New engine rebuild not running well at all at low speed

I fired up my newly rebuilt (rings, valves,) 850 yesterday and was extremely pleased that it started on the first kick. However, after running for a while it got decidedly cranky. It wouldn't idle at normal speeds, seeming to need at least 1500 RPM to keep running. Revving the engine to the 3000+ Rpm range was fine but down low it was a mess. The Amal carburetors were untouched during the rebuild and had been working perfectly prior - idling fine at 700 RPM. I set the timing (Tri spark) correctly and when revved up, strobed it to 31 degrees, which works well here due to the altitude. Again, prior to rebuild it would idle smoothly at 700 RPM.

I pulled the carbs anyway because the bike was acting like it was overfueled. I was thinking that maybe the float needles were stuck or whatever, having been sitting on the bench (drained) for about a month. But everything was fine. I thought about the possibility of a vacuum leak but the equalization tube between the carbs was OK and connected. I have new heat-insulating gaskets between the int manifolds and head. I didn't put any sealant on them - the old ones had sealant on the head side of the gasket. I didn't spray anything on the joint while the engine was running to check for a vac leak because I didn't think about it at the time but visually the joint looked fine - though I realize that's no "test."

The carb mixture adjusting screws work properly, even when "idling" at fairly high RPM, the screws will adjust the mixture appropriately. It is hard for me to imagine it is a carb issue - I recently went through them completely - new needles/jets, rebuild kits, etc and AGAIN, a month ago they idled perfectly at 700 RPM and did so briefly yesterday. There was sufficient fuel in the tank.

Everything directly involved with the rebuild seems to be in good shape - valve clearance is correct, compression is within 1 psi on both cylinders.

Any ideas what I may be missing? As I said, I'm stumped because initially it ran perfectly but went south after running for a few minutes. If I rev it up it sounds strong.

Is the battery fully charged and load tested? Is the gas fresh and new? Are the plugs new? Be completely sure of all three. If yes to all three than check the ground wires and the switching connections. If you have a kill button check it. The Trispark is very good down low but it still needs good gas, good plugs, good wiring. As described it sounds like electrical weakness. With the air circuit sensitive to adjustment and the jetting also prior proven sounds right but are you using the choke? And if you have removed them does it behave better or worse with just ticked carbs? A very well tuned carb will need a choke till the chambers are hot enough to deal with cool unleaded.

I don't care how good those plugs look, I'd replace them with a fresh, gapped set.

Stale gas (sat in the tank while you did the work)?

Air leak? - tried spraying WD-40 around the intakes while running?

All good points - I'll check batt voltage this morning. THe gas did sit over the month though the gas here in Mexico is much better than the gas now used in the US and it doesn't deteriorate (no ethanol...yet) rapidly - looks and smells like the gas we USED to have in the US! That being said, I'll get new gas though it ran fine initially on that gas. Plugs were new just prior to the rebuild - hard for me to think they are an issue but new ones sure couldn't hurt.

All wiring is new (new harness a couple of months ago) and good solid additional heavy ground wiring from frame/engine/batt pos terminal. Kill switch (new "honda" switchgear from Colo Norton works installed 2 months ago) is fine - good solid blue spark at plugs.

I'll advise what I find as I proceed...

thanks

Mike
If you need 1500 + to keep it running, sounds like a blocked idle jet or jets. Carbs and tank will have been disturbed and it only takes a tiny piece of crap to foul those little buggers. Could be an air leak. Did you check the intake balance pipe, I've had that blow off after a backfire and not notice it though it takes a bit more than 1500 to keep it running!

Mike,
You said it ran fine for a while. How long was that? Have you tried to run it since it's had a chance to cool down? Revving the engine with no load doesn't really tell you much. I would try to get it out for a ride if possible. You can always tell if one cylinder is wonky and the other pulling it's load, or if they're both crapping out. If they are both acting up I would skip the carbs. The chances of them both all of a sudden having sticky floats or blocked jets is pretty slim. If it were me I'd try running down the ignition.Check the battery. Use a jumper to power up the ignition directly to the battery post(s). That would be both positive and negative leads to the TriSpark. Bypass all of the harness. Double check the wiring from the TriSpark to the coil. Have you got a multimeter?

It SEEMS to be ok now...

It SEEMS to have been a vacuum leak at the new phenolic heat insulator gaskets between manifold and head. I did not actually check while running yesterday because I didn't think of it at the time but I pulled both carbs this morning and applied some permatex grey to the head-side of the gaskets, checked that the carb manifolds were flat (they were from the last time I surfaced them), bolted everything up and after a couple of minutes of re-adjusting the slide stop screws and the mixture screws, it settled down to a smooth 7-800 RPM idle.

I used the term "SEEMS to have been a vacuum leak" because I also checked batt voltage and this morning it showed 12.30V. With the ignition on, it dropped to 12.15. I was under the impression that the trispark was OK with that sort of voltage, however to be sure I charged the batt for a while until it was holding 12.6 after the charger was disconnected. THen reinstalled the carbs and started the bike.

SO I suppose it could have been a voltage issue with the batt/trispark. I was under the impression that the trispark was OK with even less than 12v (unlike the Boyer) but maybe not? The oem points would fire just fine at 9V!

I can't ride it at the moment - the traffic is so heavy here in Mexico City that it's basically impossible to do much more than idle around. But I'll take it out this weekend - if you get out and back before around 10AM on the weekend, especially Sunday, you can actually get beyond 2nd gear!

Thought I'd add one more bit of info re voltage and the Trispark.

Late this afternoon I started up the bike, hooked up a multimeter to the batt, turned on the headlight and, at idle, watched the voltmeter to see if/when the Trispark might start acting up as the voltage dropped. The batt voltage dropped to 11.6 but that's as low as it would go - at around 700RPM the alternator was providing enough charge to keep it from going below that. The trispark didn't miss a beat at that voltage.

Fully cognizant that 95% of carb problems are electrical, this one still sounds like a carb problem....idle jets clear? Air screws working?

I've found that my Combat doesn't start as readily with the Tri-Spark set at 31 degrees--28 degrees is better but starting still isn't as good as with the analog Boyer.

Could your float bowls possibly be marginally loose? I finally got Allen screws and carried a key with me to keep them cinched up--they would vibrate loose and wouldn't idle below 1500 rpm.



Tim Kraakevik
kraakevik@voyager.net

You might want to check or just replace the balance hose between the carbs. Sometimes this gets twisted up when the carbs are off the bike and if the rubber is old it could crack. An air leak would cause high idle. But looks like you found the problem. Nice looking bike.

Russ