750 Combat top speed problem (2015)

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needing said:
Low fuel level in the float bowl doesn't lean the mixture UNTIL the main jet is exposed. As the float drops more fuel flows through the float needle seat to replace fuel consumed unless there is a restriction in the fuel delivery
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The pressure pushing fuel through the main jet is a combination of the head of fuel above the jet (fuel level in the float bowl) and the vacuum signal being applied to the jet.
Reduce the vacuum signal and it flows less.
Reduce pressure from the fuel head (lowering fuel level in the bowl) and it will flow less too. We're talking about approx 6 mm lower fuel level, when the needle is fully open (compared to when it's closed).

Hobot's slot beside the needle is a good idea. I've drilled 1/8" holes in the bowls at the same position.
 
Did you really mean 33% leakage at 10 psi, and it's going past the rings? That sounds like stripdown time. I would have thought max 10% leakage at whatever is the recommended pressure would be OK on a street engine in reasonable condition, but please correct me I have understood your figures wrongly. Have a look at Jim Comstock's leak down test, where he mentioned that 7% leakage was good: new-leakdown-results-t21149.html[/quote]

Thinking about it more and also after seeing a UTube how to do the leak down test, my 33% leakage is an issue and likely the culprit. Another symptom I have is excess oil leaks at the head, particularly the exhaust valve covers when at higher speeds such as 70mph on the freeway....likely excess pressure in the crankcase from blowing by the rings. I mentioned already have the XS reed valve in the engine breather. I'm going to retest the lead down to confirm both cold and hot engine.
 
ntst8 said:
illf8ed said:
Back again. Lucas RITA sorted out and getting one timing mark....
Click to expand...
Out of interest, what did you change to get the single timing mark??
Click to expand...

The rotor wasn't centered in the taper of the camshaft so the gap on the two points was different - closer fires earlier. I carefully attached it again and checked to confirm the gap was the same on both trigger points.
 
illf8ed said:
Thinking about it more and also after seeing a UTube how to do the leak down test, my 33% leakage is an issue and likely the culprit. Another symptom I have is excess oil leaks at the head, particularly the exhaust valve covers when at higher speeds such as 70mph on the freeway....likely excess pressure in the crankcase from blowing by the rings. I mentioned already have the XS reed valve in the engine breather. I'm going to retest the lead down to confirm both cold and hot engine.
Click to expand...

If the rings are badly worn, it will likely be burning oil and you could have someone follow you to see if there is smoke on acceleration. You say it runs cleanly through the gears but won't pull 4th. No unpleasant or unusual noises?

Even if your leakdown tests turn out not be definitive, you at least know the valves are sealing, and a simple compression test might now be a good idea (if you don't have, beg or borrow a gauge!). If the figures are within the range of acceptability and it's not burning oil, you could then rule out the rings. It would be a shame to strip the top end if it turned out to be something external.

Whatever the problem is, it must be fairly severe if it won't pull 4th.

P.S. Do you have two six volt coils with your RITA?
 
Ugh we would expect bore/ring wear to smoke as well as blow by oil out head seams but my lawn mower began loosing power over a few weeks till barely runing to return to shed and then not enough compression to start again but it fooled the snot out of me as did not smoke. I would not trust a 10 PSI leak down test but would a regular ole compression test for incentive of show stopping investigation. Btw it was my hunt to recover lawnmower bores long term a couple yrs ago while Canaga still had Peel engine apart that made Bore Tech a common name here.
 
Hi X-file.
The pressure created by the fuel tank being above the carbies is how the float level is maintained (i.e. no restriction in the fuel supply system). It does not 'push' fuel through the main/pilot jets until the tickler holds the float down to increase the float bowl level. This is when float level pressure 'pushes' fuel though the jets.

When the motor is kicked over (and running), Vacuum alone 'pulls' fuel past the jets.
Please review your understanding of Bernoulli's principle:
http://www.cdxetextbook.com/assocStudie ... ciple.html
http://www.princeton.edu/~asmits/Bicycl ... oulli.html
http://en.m.wikipedia.org/wiki/Bernoulli's_principle

Float level is set at 5mm below the float bowl lip on my carbys without 'lean' issues even when leaning since (I repeat...) 'lean' air:fuel mixture is only achieved when the main jet is uncovered (or the jet orifice is too small for WOT wide-open-throttle).
Ta.
 
daveh said:
illf8ed said:
Thinking about it more and also after seeing a UTube how to do the leak down test, my 33% leakage is an issue and likely the culprit. Another symptom I have is excess oil leaks at the head, particularly the exhaust valve covers when at higher speeds such as 70mph on the freeway....likely excess pressure in the crankcase from blowing by the rings. I mentioned already have the XS reed valve in the engine breather. I'm going to retest the lead down to confirm both cold and hot engine.
Click to expand...

If the rings are badly worn, it will likely be burning oil and you could have someone follow you to see if there is smoke on acceleration. You say it runs cleanly through the gears but won't pull 4th. No unpleasant or unusual noises?

Even if your leakdown tests turn out not be definitive, you at least know the valves are sealing, and a simple compression test might now be a good idea (if you don't have, beg or borrow a gauge!). If the figures are within the range of acceptability and it's not burning oil, you could then rule out the rings. It would be a shame to strip the top end if it turned out to be something external.

Whatever the problem is, it must be fairly severe if it won't pull 4th.

P.S. Do you have two six volt coils with your RITA?
Click to expand...

Using stock Lucas 6v coils with the RITA and have been running this way since 2002 - Boyer microdigital with same coils before that back to 1998. Before that the machine was dead from 1977 to 1998 :wink:
 
illf8ed said:
Using stock Lucas 6v coils with the RITA and have been running this way since 2002 - Boyer microdigital with same coils before that back to 1998. Before that the machine was dead from 1977 to 1998 :wink:
Click to expand...

OK :) , just thought it was worth mentioning because, years ago, I connected the wrong coils to a RITA and wondered why the performance was way off. So did this problem come on gradually or was it after you changed something? If something was changed, that might help to provide a clue.
 
Even if your leakdown tests turn out not be definitive, you at least know the valves are sealing, and a simple compression test might now be a good idea (if you don't have, beg or borrow a gauge!). If the figures are within the range of acceptability and it's not burning oil, you could then rule out the rings. It would be a shame to strip the top end if it turned out to be something external.


What is considered the acceptable range when doing a compression test on a standard 750 commando.

Dennis
 
I have the tri spark set up in two 750 commandos,sourced it from Matt @ CNW.It is bullet proof...set it and forget it,by far worth every penny,lots of peace of mind too.It is a simple install with Matts...as usual very simple and detailed install sheet.
 
zefer said:
I have the tri spark set up in two 750 commandos,sourced it from Matt @ CNW.It is bullet proof...set it and forget it,by far worth every penny,lots of peace of mind too.It is a simple install with Matts...as usual very simple and detailed install sheet.
Click to expand...

I agree, however my RITA is also bullet proof and isn't going to cost me $300 because I already have it.
 
Am I mistaken or is it the trispark that seems to quite often give up the ghost after a few years and maybe 15-20,000 miles of riding? From reading this forum that is the impression I get. If your annual mileage is low, you might never experience a problem.
The Boyer, aside from the avoidable broken pickup wire problem, seems almost trouble free as does the Pazon. Also not a lot of complaints about Lucas Rita, other than it draws quite a lot of power.
I think Ludwig was one who had trouble with and eventually gave up on Trispark, but then he probably rides more than most. I guess it is one of those products that is good until it isn't.
This is just my interpretation of the Electronic ignition troubles reported on this site and is not in any way a controlled study of some sort.
A search of the site under "Boyer problem" then "Trispark problem" etc is quite telling. Of course one needs to spend the time to read thru each thread to determine if it really was the ignition at fault .

Glen
 
worntorn said:
Am I mistaken or is it the trispark that seems to quite often give up the ghost after a few years and maybe 15-20,000 miles of riding? From reading this forum that is the impression I get. If your annual mileage is low, you might never experience a problem.
The Boyer, aside from the avoidable broken pickup wire problem, seems almost trouble free as does the Pazon. Also not a lot of complaints about Lucas Rita, other than it draws quite a lot of power.
I think Ludwig was one who had trouble with and eventually gave up on Trispark, but then he probably rides more than most. I guess it is one of those products that is good until it isn't.
This is just my interpretation of the Electronic ignition troubles reported on this site and is not in any way a controlled study of some sort.
A search of the site under "Boyer problem" then "Trispark problem" etc is quite telling. Of course one needs to spend the time to read thru each thread to determine if it really was the ignition at fault .

Glen
Click to expand...
Hi Glen
Enjoyed your rational thinking.
Having worked for 45 years in industrial electronics, I May have picked up on a few things. Back ground... 20 years USAF ground radar master tech. Then 25 years (now usaf retired? what a joke) in high power electronics radar manufacturing.
All history now. totally retired 1n 2013 (still a joke)
Heat is the enemy. period!!
If you can control the heat dissipation you control LIFE (of electronics). Any electronics (semi conductors) can live, running continuously, until we are all in the grave, if you control heat.
Back to MC ignitions.
The use of these electronic devices comes with additional application problems as you mentioned. I now have to give though to trispark really having heat really under control?
May be an experiment coming?
Dave
 
Dave, you caught me right at the perfect level with my heart meds, floating between howling at the moon and crying for no reason. :mrgreen:

Glen
 
I wanted to get into the engine before posting more on the leak down test results. Leak down had much worse results this time 70%-80% leakage after warming up the engine. Head and cylinders off now. Question: can a leaking head gasket (copper) result in air leak noise in the oil tank during the lead down? No noise at all at the exhaust pipe or carbs. I have not yet measured piston to bore clearance, but did take the top compression ring off the right piston inserted into the bore and looked at ring gap from bottom to top. The gap looked the same and good, so assuming the rings and bore are still good (initially). I don't have a micrometer for measuring the bore accurately and using a dial caliper the pistons show minimum wear top to bottom (+.020 bore). Piston have 35,000 miles and the rings have been replaced once with around 15,000 miles on them.
 
More than one of my many head gaskets allowed blow by to enter push rod tunnels into crank cases then exit breather into oil tank.

To 'adequately check bores w/o fancy guage put in new rings you know should have correct gap for the bores, std, .020, 040 over etc and see if gap ok or too wide d/t worn bores. Can't use old rings as they should wear more than bores. IIRC ~ .120 gap is expected.
 
illf8ed said:
I wanted to get into the engine before posting more on the leak down test results. Leak down had much worse results this time 70%-80% leakage after warming up the engine. Head and cylinders off now. Question: can a leaking head gasket (copper) result in air leak noise in the oil tank during the lead down? No noise at all at the exhaust pipe or carbs. I have not yet measured piston to bore clearance, but did take the top compression ring off the right piston inserted into the bore and looked at ring gap from bottom to top. The gap looked the same and good, so assuming the rings and bore are still good (initially). I don't have a micrometer for measuring the bore accurately and using a dial caliper the pistons show minimum wear top to bottom (+.020 bore). Piston have 35,000 miles and the rings have been replaced once with around 15,000 miles on them.
Click to expand...

The pistons are not perfectly cylindrical but maybe you meant you measured the bores? I assume you will measure the piston to bore clearances at the bottoms of the piston skirts with feeler gauges, both at the bottom and top of the bores. Even if you have no new rings, what are the existing piston ring gap values you get at the top and bottom of the bores? Also, are the rings a good snug fit in each of their grooves? What does the camshaft look like? Your leak down test indicated that the valves were sealing, but now you have the head off, double-check by pouring some gasoline into the ports and look for seepage into the combustion chambers.

Do you think it's a bit strange that you haven't discovered firmer evidence for the bad leak down test figures?

Can you post some pics of the pistons, bores, spark plugs and combustion chambers?

Now that you have the top end off, I suggest you check everything, even if you had the head rebuilt recently.
 
daveh said:
illf8ed said:
I wanted to get into the engine before posting more on the leak down test results. Leak down had much worse results this time 70%-80% leakage after warming up the engine. Head and cylinders off now. Question: can a leaking head gasket (copper) result in air leak noise in the oil tank during the lead down? No noise at all at the exhaust pipe or carbs. I have not yet measured piston to bore clearance, but did take the top compression ring off the right piston inserted into the bore and looked at ring gap from bottom to top. The gap looked the same and good, so assuming the rings and bore are still good (initially). I don't have a micrometer for measuring the bore accurately and using a dial caliper the pistons show minimum wear top to bottom (+.020 bore). Piston have 35,000 miles and the rings have been replaced once with around 15,000 miles on them.
Click to expand...

The pistons are not perfectly cylindrical but maybe you meant you measured the bores? I assume you will measure the piston to bore clearances at the bottoms of the piston skirts with feeler gauges, both at the bottom and top of the bores. Even if you have no new rings, what are the existing piston ring gap values you get at the top and bottom of the bores? Also, are the rings a good snug fit in each of their grooves? What does the camshaft look like? Your leak down test indicated that the valves were sealing, but now you have the head off, double-check by pouring some gasoline into the ports and look for seepage into the combustion chambers.

Do you think it's a bit strange that you haven't discovered firmer evidence for the bad leak down test figures?

Can you post some pics of the pistons, bores, spark plugs and combustion chambers?

Now that you have the top end off, I suggest you check everything, even if you had the head rebuilt recently.
Click to expand...

I will need to wait until next weekend to get any more data, but what you mention is the direction I'm going.
 
If you swap the 6V coils for 1890 gas inductors , youll notice the improvement .

Useing Lucas 6 V copils you deserve all you get . They wouldnt even get a cat to jump .
Were ratted most useless ( after the lucas 12 V ) in the old ' great coil test ' in cycle magazine . for their dewcati racer .
k mart $ 10 Chev coil rated best . And used to win Daytona on the SS Duke .

So NOW you KNOW . But dont listen to ME . ! :P

There should be a LUCAS 6 V COIL tossing Competition . :twisted: :wink:

A BLUE WHITE 15 mm spark is ADEQUATE . less isnt . So There .

probly acts like its leanish & flatish . Spark Breaking down under pressure . B.M.E.P. when it wants to go. So it doesnt . DONT ask me how I know . :( :x
 
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