750 amal tuning tips ?
- Thread starter stu
- Start date
- Status
- Joined
- Nov 11, 2013
- Messages
- 5,396
L.A.B. said:
Shouldnt that be verifiable if only one of the plugs were sooty? If both are sooty, seems like the Boyer is the more likely suspect? (and much easier to check)
htown16
VIP MEMBER
- Joined
- Apr 29, 2009
- Messages
- 1,512
Put a piece of tape on the handle bar by the throttle and mark a reference on the throttle and 1/4, 1/2, 3/4 open on the tape. If the problem is happening at the same throttle position in different gears, which will of course be different rpms, it is likely carb. Now try the same rpm, say 2500 or 3000 in different gears, which will of course be different throttle positions, if the problem is occuring at the same rpm in different gears, it is more likely electrical.
- Joined
- Nov 11, 2013
- Messages
- 5,396
htown16 said:
Like!
needing said:
I know this is backtracking a bit, but some of this isn't right.
Carbs are designed so they DON'T respond to manifold vacuum, except for the idle circuit.
Otherwise, when the throttle was closed and manifold vacuum was strongest it would try to deliver the most fuel.
And when the throttle was open and the manifold vacuum was lowest, it would deliver the least fuel.
Exactly the opposite of what is required.
Think of the old style carbs in cars, with only a butterfly to throttle things.
The Bernoulli effect of the airflow across the jets IS what puts fuel mist into the airflow through the ports. That part is correct.
No airflow = no fuel delivery.
Lets keep it correct....
- Joined
- Oct 19, 2005
- Messages
- 18,978
Needing got it right just in common man lingo as its the pressure drop in manifold that creates the breeze sheeze. Btw real old school carb experts do all the initial stages of tuning w/o the main jet installed before working it up to too rich then reducing a step. Peel will use air density flow not venturi sucksion. I believe Needing is more than up on pressure and flows for any terminology correction from blow by to intake sucksion pulses but still a jerk online.
Few days ago trying to take apart crusty old Kohler 18ph Magnum first by .223 and next one more delicately by 1 hp 12 inch angle grinder, the carb to manifold gasket has a stamped out saw tooth turbine like intrusion into air flow just like Peel did when she woke up to first piss me off as about ran out from under. Peel was on the rich side prior.
Few days ago trying to take apart crusty old Kohler 18ph Magnum first by .223 and next one more delicately by 1 hp 12 inch angle grinder, the carb to manifold gasket has a stamped out saw tooth turbine like intrusion into air flow just like Peel did when she woke up to first piss me off as about ran out from under. Peel was on the rich side prior.
hobot said:
Needing didn't get it right - or carbs would supply more fuel under BIGGER vacuum, and less under less manifold vacuum.
Exact opposite of what happens.
Its the AIR FLOW through/over the venturi that gives the game away. That bit is right. 2 contradictory statements.
Read up on that Bernoulli effect he even mentions...
This is not necessarily so. It depends on throttle position, not RPM. Under moderate acceleration or if running at a steady 2500-3000 RPM, true, main jet has little influence. But consider this. WOT in each gear yanks the slides up as far as they go and the metering needles are consequently lifted out of the main jets as far as they can be. You are running on the mains no matter what the RPM, before, going through, and after 3000 revs.[/quote]
The main jet size is pretty much irrelevant unless you are doing a blast down a long straight road with the throttle wide open. For most commuting you will be metering the mixture with the needle and needle jet with the throttle part closed. As long as the mains are rich enough you will stay out of trouble. The height of the needles is critical - too high and acceleration will be sluggish - too low and the motor will cough as you go up and down through the gears. If the choke circuit is not operating correctly, it is difficult to tune the needles and needle jets to make sense.[/quote]
Several years ago I watched John Healy, a Triumph guy, give a tech. session on Amal tuning at mid-Ohio. He said that he had removed the main jets altogether from a bike and run it that way as an experiment. According to John you couldn't really tell that anything was wrong riding around a parking lot. That's how much effect main jets have at low throttle opening.
The main jet size is pretty much irrelevant unless you are doing a blast down a long straight road with the throttle wide open. For most commuting you will be metering the mixture with the needle and needle jet with the throttle part closed. As long as the mains are rich enough you will stay out of trouble. The height of the needles is critical - too high and acceleration will be sluggish - too low and the motor will cough as you go up and down through the gears. If the choke circuit is not operating correctly, it is difficult to tune the needles and needle jets to make sense.[/quote]
Several years ago I watched John Healy, a Triumph guy, give a tech. session on Amal tuning at mid-Ohio. He said that he had removed the main jets altogether from a bike and run it that way as an experiment. According to John you couldn't really tell that anything was wrong riding around a parking lot. That's how much effect main jets have at low throttle opening.
ewgoforth said:
Indeed. I've heard stories of folks finding their bikes weren't even fitted with main jets, or that one had fallen off/down.
And some had been riding them for years like that ?
Giving it full throttle from low rpm is not a common riding style ?
Sounds a little beserko.....
a quick update - set the carbs back as they were with lowered needles on the 230 mains, brushed off plugs and fired up. LH cylinder (always v sooty plug) weak at the exhaust (not firing) with a trace white smoke at exhaust on revs. RH cylinder (usually dark/tan plug) pumping out the exhaust like a champ. Since I revved up the other day to 5k rpm for the strobe and the numbers roughly correspond with LABS boyer curve i'm now wondering maybe an ignition problem on the left cylinder ? Checked the birdsnest of wires for breaks and swapped the HT leads over - LH still weak so maybe the coil (PVL 6v) ? Could've been weak a while and now given up the ghost ?
stu said:
Can you tweak the throttle cable for the LH cylinder, and verify that it will rev up on its own, at all ?
Swapping plugs/leads/coils side-to-side can verify if a component there is dud.
Performance on the road may be poor !! if one cylinder isn't doing anything....
Not so easy to verify if ignition system is conking out on one side.
The strobe would at least tell you if it is still sparking.
. . well . . after troubleshooting the electrics and carbs to death and being sidetracked by a loose electrical connection i started into the timing side. Laying the support plate across the end of the intermediate gear and cranking the camshaft chain round i straight away found a lot of slop. Readjusted the tensioner slipper at the tightest point in the chain back into spec but still an inch of play in other parts of the chain so a new one is on order. Looking closer i noticed only 9 rollers between the teeth marked with dots (should be ten ?) so now i'm thinking i found the root problem to my poor running. Cranking the intermediate gear round to 6 oclock (IG paint marks sandwiching a tooth on the timing pinion) definitely sets up only 9 rollers along the top but the illustration in the workshop manual is a bit vague. So, dots (timing marks) correspond to teeth, 10 rollers between the teeth right ?
- Joined
- Nov 20, 2004
- Messages
- 20,351
- Status
