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General Category => General Discussion => Topic started by: Ambo_bubba on August 17, 2025, 08:45:33 AM
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Getting my 71 Ambo back on the road after extensive restoration. Put a Cycle Garden big bore kit on it, and have ridden about 100 miles now. I originally used the up sized jets from cycle garden, but noticed that I couldn't get my idle to steady out any lower than 1,500 rpm. Other than that it ran great.
I decided to tackle the idle issue after the 100mile break in maintenance. Replaced the choke plugs, and readjusted everything and tried again. At this point the idle was still way high and I noticed my throttle and choke cables were adjusting way too high. After correcting that I started it back up again and went to work adjusting the idle. I noticed the fuel mixture screw only made a change when it was just about closed, so I switched the pilot jet for a stock size pilot jet. That's when issues started. After reinstalling the carbs and starting it back up, my left cylinder was dead. Right side idles fantastic on its own, but zero juice out of the left and the intake manifold was cold cold! Looked like the carb was dumping fuel and soaked the spark plug (ultimately it fouled it).
I took the left carb off and checked everything, didn't find any obvious faults, but to give it a whirl i swapped the left and right spark plugs and the left cylinder fired to life. So I put in new plugs, synced the carbs, and set the mixture screws to 1.5/1.75 turns and started her up. Right cylinder running strong, nothing from the left. If I all but close the left carbs mixture screw, it starts to sputter to life. I was hoping to take it for a quick spin to maybe burn off any residual unburnt fuel but no dice. Right cylinder runs great while the left keeps sputtering. Looking at the plugs, right is running lean and left is way rich.
My guess, based on the idle mixture screw bringing the cylinder to life when closed, is that the idle circuit is dumping fuel into the cylinder. Any thoughts on what the issue could be and what needs adjusting in the carb? Floats and float needle are new, as are the choke plugs, needles, jets, etc. carbs were cleaned thoroughly during the restoration.
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Check your choke plungers for deterioration. Ethanol kills the rubber seal, they develop cracks and leak like you have the enrichener open. Common to all plunger choke carbs.
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The plungers are brand new and look good
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Check float for leakage? Rare but happens.
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I'll pull the carb out and double check. This is where my head went to at first. How do you adjust the float the height on these carbs? The float needle is new and the seal looks good, but on the floats I can't find anywhere that would let me push them further up or down.
Thanks!
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If you replaced the original floats with the crappy white ones, put the old ones back in and set the float level to 24 mm +/- .5 mm. The white floats will crack and fill with fuel.
I had a customer's Ambo here that wouldn't run worth a crap - he had replaced the original "choke" springs with new ones. The new springs were too short and didn't seat the "choke" plungers well.
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Take out the new crap and put the old stuff in is becoming a trend on this bike :cheesy:
I'll check the choke plungers again and swap the floats and report back. Thanks Charlie!
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How do you adjust the float the height on these carbs? The float needle is new and the seal looks good, but on the floats I can't find anywhere that would let me push them further up or down.
You need to bend the small metal tab that sits against the end of the float bowl valve plunger to adjust the height of the floats.
You certainly seem to know what you are doing in general, but you might profit from looking at the Dellorto manual that was written for the later series of carbs. Much of the information pertains also to the VHB's. EDIT: Here's the colorized the better, black-and-white, version:
https://www.thisoldtractor.com/mg_manuals/dellorto_manual_a_guide_1.1.pdf (https://www.thisoldtractor.com/mg_manuals/dellorto_manual_a_guide_1.1.pdf)
http://www.ducatimeccanica.com/dellorto_guide/dellorto_3_7.html (http://www.ducatimeccanica.com/dellorto_guide/dellorto_3_7.html)
One important idea throughout that manual is working in series from idle to full throttle as you adjust the mixtures.
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That manual is exactly what I need, thank you!
I generally know enough to get myself in trouble, I then find I generally don't know enough to fix what I did wrong :laugh:
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How old are the carbs?
I just went through idle issues in my T3.
Finally found the problem was air leaking by worn slides.
Would have never suspected.
120,000 miles on the carbs.
Replaced (and reseated) slides and all is good.
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Well more tinkering and still nothing...
Floats are set at 24mm even and I verified them working by trying to single them in a bowl of water for 30 minutes, no leaks.
Then I went back and tried to start it, nothing, pulled the plug out and it looked brand new and wet, too much gas.
Now, to sync the carbs, since I don't have vacuum ports, I use the drill bit method where I used a 9/32nd drill bit to set the height on both carbs. And was then making adjustments together.
In a fit of desperation while the bike was idling on the right cylinder alone I started screwing with the screws independently on only the left carb, with the idle screw cranked down, the left cylinder once again started firing to life...
So either I have a need for different fueling between both cylinders (I don't think so, at least on a new engine it shouldn't be so significant as to cause an engine to go completely dead) or the carb is fueling differently.
I am confident at this point in the floats operating correctly and the choke plungers closing all the way. At least I think if I had a problem there it would show as shitty running, not no running at all.
I have half the idea to try to get the bike to idle on the left cylinder alone, and then going back and vacuum syncing the carbs after that (I don't have vacuum ports but I do have a vacuum reader for itbs that I stick on there with the air box off. Not sure if this is smart
Or, potentially the idle jets I received are different despite being marked 45? I'm thinking about swapping back in the 50s and going from there, at least I knew those were good.
Any other thoughts or methods? Thinking on other issues I don't think it could be timing as the bike starts within a second every time, and compression feels about the same but I haven't hooked up a tester yet
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How old are the carbs?
I just went through idle issues in my T3.
Finally found the problem was air leaking by worn slides.
Would have never suspected.
120,000 miles on the carbs.
Replaced (and reseated) slides and all is good.
That's good insight, I would never even think that the slides could be problematic. But anyways the issues seems to be that I'm getting way to much fuel at idle, rather than too much air. If I independently crank in the idle air screw on that cylinder I can get to start sputtering to life (at this point the screw is adjusted almost all the way in. Or if I completely close the mixture screw. Other than that the cylinder is completely dead.
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So now you've been through the carbs fairly thoroughly- it's time to look at the ignition. My '85 LeMans was problematic for some time on one cylinder- don't recall which- but the short of the long story was that the hot-to-coils wire between the two coils had a marginal connection at the ring terminal, so one coil was current-limited. Of course, it's nearly impossible to test at rest but you might have a look just to see that it's secure, and if it's not too hard to access clip a jumper across the hot lead joining the two.
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That's good insight, I would never even think that the slides could be problematic. But anyways the issues seems to be that I'm getting way to much fuel at idle, rather than too much air. If I independently crank in the idle air screw on that cylinder I can get to start sputtering to life (at this point the screw is adjusted almost all the way in. Or if I completely close the mixture screw. Other than that the cylinder is completely dead.
I would look at the needle valve seat for corrosion. I bought a second set of old old crusty VHBs for comparison and yep they had the same problems. Evidenced by fuel weeping around and out the idle air mixture screw overnight and flooding while running. If you know a jeweler maybe he will polish your seats like new.
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I would look at the needle valve seat for corrosion. I bought a second set of old old crusty VHBs for comparison and yep they had the same problems. Evidenced by fuel weeping around and out the idle air mixture screw overnight and flooding while running. If you know a jeweler maybe he will polish your seats like new.
One can polish their own seats. Cut one end off of a Q-Tip and chuck it up in a drill. Apply valve lapping compound. Insert the Q-Tip into the seat and spin it a few seconds, pressing it lightly into the seat. Remove and clean the seat thoroughly. Done.
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I would look at the needle valve seat for corrosion. I bought a second set of old old crusty VHBs for comparison and yep they had the same problems. Evidenced by fuel weeping around and out the idle air mixture screw overnight and flooding while running. If you know a jeweler maybe he will polish your seats like new.
This is where the mixture screw needle sits or where the actual needle on the throttle slide sits? Or where the float needle sits?
I'll take a look at all three this evening
Edit, I'm at the bike now, I found out I left the Petcock overnight... Noticed the weeping around the idle mixture screw, as well as an airbox full of gas and a small puddle under the bike...
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Issue found... I pulled both carbs and compared the jets, on the left the carb the pilot jet was looking quite like a main jet. It was marked 45 but the 1 must have been left off the markings. I put the original pilot jets I had for the bike back in and will attempt a start as soon as it's all back together... Hopefully that was all
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Started the bike, problem solved :cool:
I'm a little annoyed I took the carb apart 3 times and didn't notice, but glad it was a simple fix. Thanks everyone for their help!
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Started the bike, problem solved :cool:
I'm a little annoyed I took the carb apart 3 times and didn't notice, but glad it was a simple fix. Thanks everyone for their help!
Don't feel bad, I still do it on VHBs and V65 carbs sometimes. Just a few weeks ago I had the 105 main and the 42 idle jet in the wrong places in my right-side V65SP carb. Didn't run too well like that. :grin:
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Since you had the gas leak. Check your oil!!!!! If there was gas in the air box, there is likely gas in the oil.
Tom
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Since you had the gas leak. Check your oil!!!!! If there was gas in the air box, there is likely gas in the oil.
Tom
I'm not a huge fan of the oil fill plug location on the bike as it makes for an awkward angle to fill with a funnel, but it does work well for giving the oil sump a good whiff!
Oil change is in order for tomorrow