Starting issues on rebuild

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davecox2

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Hi folks!

My 1974 850 rebuild is coming along great, and I am to the point of test firing the motor. I'm able to get it to fire, but it will only run for about ten seconds and then die, even if I give it some throttle. I am getting plenty of fuel trickling when I tickle the carbs (original twin Amals, rebuild by a local British shop), so I know fuel is getting there, but it doesn't seem to want to keep feeding fuel to the motor. I have strong spark and it'll fire on the second or third kick (PowerArc from Old Britts) Does anyone have any tips? I feel like I'm missing something simple.

Also, has anyone ever had an issue with the rocker oil feel lines failing? Twice now I have had the hose blow off of the banjo bolt, spilling oil everywhere. I'm not sure if these two issues are related, but thought I'd run it by the experts!
 
did you check the float heights in the carbs? sounds like fuel. Maybe once it fires, stay off the throttle and keep the ticklers held in on the carbs and see if it continues to run.
 
Try starting the bike with the choke on.

Have the carbs been set up initially (ex. balanced and 1-1/4 turns out on the idle jet screw)?

As for the rocker feed lines, I broke one and heard plenty from others who have broken theirs. They typically break and don't slide off. I convert to the stainless steel braide lines as I have no faith in the black plastic. They were sub marginal when new and who knows what they are now after 40 years of shelf life.

The rocker feed blowing off and difficult starting are not related.
 
The bike starts harder with the choke on for some reason. The carbs were (supposedly) set up by the guy who rebuilt them.

My rocker feed line was a brand new one, though not stainless. I have a replacement coming, and hope I just had a defective one!
 
I would have assumed they were clear, but I'm at the point where I can't take anything for granted. What's the best procedure for checking this?
 
Try opening up BOTH gas taps and click open the gascap and let it rest for this first startup test. Plugged idle jets are typical , banjo wire or micro-bit drill attached to spray-tube hand-held spin in the oriface works to clear hardened fuel crud. Consider investing in braided stainless rocker feed line as the nylon black plastic type will fail eventually ,usually at the most inopportune moment. You can get to home or help though by jamming cloth into the line to stop oil flow to head. I've had to do this before to go the 10 min. or so drive home as the head is overoiled anyways and there was no harm done. But I diverge ,get it running.
 
It's best to know you are coming from a known and these are easy things to check before tearing into it.

Make sure the air bleed screw are 1-1/4 turns opened. This is the standard starting point.

Are both carbs idle screws set identical? In other words, are both slides raised the same with the idle screw?

Are both carbs slides rising together? In other words, are both cables activating the slides equally?

Below is a handy schematic of the AMAL for your reference.

http://www.oldbritts.com/amal_tun.html

As concours stated above, start checking and cleaning the idle circuit.
 
I went ahead and ordered a stainless oil feed line to eliminate that particular issue. Plus, it'll look great!

I've double checked a few suggestions:

Dances with Shrapnel said:
Make sure the air bleed screw are 1-1/4 turns opened. This is the standard starting point.

Are both carbs idle screws set identical? In other words, are both slides raised the same with the idle screw?

Are both carbs slides rising together? In other words, are both cables activating the slides equally?
Click to expand...

All of those look good on my carbs.

Dances with Shrapnel said:
As concours stated above, start checking and cleaning the idle circuit.
Click to expand...
I am going to look through the Bushmans link that DogT posted, but does anyone have any quick tips on cleaning the idle circuit?

Oh, one more thing... when starting, if I keep pushing the ticklers, it will keep running. Once I stop tickling them, it lasts about ten seconds and then dies.

I appreciate all the help! Thanks!
 
Check around, I think the idle jet is .016, but check for sure. I use a guitar string, you can get a drill in that size. Mount it on something so you can handle it. It's impossible for me to see the hole, it's way in there I just have to work it in there until I feel it's gone into the hole. Then with the screw left out, shoot some carb cleaner in it and you should have cleaner coming out 2 other holes. Shoot in the other holes too and make sure everything is clear. That should do it.

Dave
69S
 
Perfect, thanks! I know what I'm doing tonight. I just saw the guitar string recommendation on the Bushmans site too. I have a basement full of guitars, so I have tons of strings available!
 
davecox2 said:
I went ahead and ordered a stainless oil feed line to eliminate that particular issue. Plus, it'll look great!

I've double checked a few suggestions:

Dances with Shrapnel said:
Make sure the air bleed screw are 1-1/4 turns opened. This is the standard starting point.

Are both carbs idle screws set identical? In other words, are both slides raised the same with the idle screw?

Are both carbs slides rising together? In other words, are both cables activating the slides equally?
Click to expand...

All of those look good on my carbs.

Dances with Shrapnel said:
As concours stated above, start checking and cleaning the idle circuit.
Click to expand...
I am going to look through the Bushmans link that DogT posted, but does anyone have any quick tips on cleaning the idle circuit?

Oh, one more thing... when starting, if I keep pushing the ticklers, it will keep running. Once I stop tickling them, it lasts about ten seconds and then dies.

I appreciate all the help! Thanks!
Click to expand...
Have you any carb cleaning experience? Moderns? The "quickie" fix involves removing the idle mixture screw, making a special tool to reach waaay in and knock the crumbs out of the passage. Some people use a guitar string (I don't know the size) and some use a tiny drill bit (#78) held in a brass rod (machining required). This MAY solve the problem. Preferred would be carbs off, so as to VERIFY flow in all passages with aerosol solvent. Did your professional carb rebuilder do this? Can you ask him?
 
As it runs a hand full of seconds, the spark is good enough, so sure sounds as listed above like too low a float level or pilot jet clogged with zine oxide crust. It possible the carb balance tube is off or leaking too or air gap in manifold. Its also possible a bad wire inside insulation vibes to shut off. Its possible the choke level is in wrong postition which may allow a start then choke down in seconds.
 
hobot said:
As it runs a hand full of seconds, the spark is good enough, so sure sounds as listed above like too low a float level or pilot jet clogged with zine oxide crust. It possible the carb balance tube is off or leaking too or air gap in manifold. Its also possible a bad wire inside insulation vibes to shut off. Its possible the choke level is in wrong postition which may allow a start then choke down in seconds.
Click to expand...

Good call Hobot... missing balance tube could do that

Also, what do the spark plugs look like? Wet? Dry?
 
The balance tube is in place. I just put new plugs in, and they still look good. I will check the manifold to make sure there is no air gap, as well as checking float levels and cleaning the carbs. Hopefully that'll do it!
 
davecox2 said:
Perfect, thanks! I know what I'm doing tonight. I just saw the guitar string recommendation on the Bushmans site too. I have a basement full of guitars, so I have tons of strings available!
Click to expand...

Be advised that clearing the jet area is only part of the solution. You need to follow the curcuit completely, from where gas is picked up in the float bowl, through the jetting and out the tiny hole in the bore/throat of the carb.

Squirt carb cleaner in all areas and witness its discharge mist out all orofaces with the use of compressed air.
 
i'm a guitar player so i used a guitar string too. .016 is the size of a G string on most common gauge electric string sets
 
BERRYMAN'S CHEM-DIP.

In the 1-gallon can.

Soak the bare carb bodies for a good 30 minutes, the warmer the better.

Poking with wires is only so good. Spray cleaners are only so good. THIS is the BEST.
 
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