This is a weird one...

[dbeardslee]

Matt - I think it was pulling enough vacumm to bring the slides up. It had enough vacuum to advance the timing. He's got the stock airbox, so it was something we couldn't see easily. Don went through them when we were swapping motors, and I'm pretty confident in his abilities - even if I do give him a hard time sometimes . I'm still looking for something to explain what we're seeing with the timing. The only things I can think of that would explain that is the timing chain being a tooth off at the crank, or a malfunctioning TCI. I checked with my friend and he's going to see if he has a 4R0 TCI that we could borrow for diagnostic purposes. Might as well try the easy stuff first .

[DGXSER]

To be honest, I only went through the mains and pilots on the carbs when we swapped motors, but Carl went back through them pretty thoroughly. I did give them a glance through when we were doing the head work and they looked clean. But I never opened up the needle valves. So I want to pull the emulsion tubes, check the pilot jets to be sure they are not the VMs, and look at the needle valve to be sure it is in the correct order.

As to the timing, my thought is that everything except the compression is driven by the crank. The timing of the spark comes form the Pick-up coils which are driven by the crankshaft through that combination of pieces we put on Saturday. So being a tooth off would/could explain it running poorly, but it would not ientify why the timing is SO far out. That one still has me puzzled.

[thewiz]

I hear what your sayin' about it NOT lookin' like fuel ...but ... any restrictive f. filters on there? How about the fuel tap tower screens? Fresh gas, right? All diaphrams seated and slides slippery?

If I had another bike handy, I might even be tempted to swap a known good set of carbs on there. If there were no change and it's not in the petcocks, at least at that point, any fuel doubts could be safely dismissed .... you could focus strictly on the other stuff.

Have you disconnected the exhaust for a little test?

[dbeardslee]

Carbs can cause a lot of problems, but they won't change the timing. Couldn't hurt to take a look at the emulsion tubes and slides - then we'll know they're clean, but I think we're still going to be looking at an ignition problem. After thinking about Don's comments, I have to agree that, even if the cam chain was off a tooth at the cam, the timing light should still illuminate the pointer at the proper mark on the timing plate. I keep coming back around to the TCI as the most likely culpret. Unfortunately, it's probably going to be several weeks before we get our hands on it again to find out.

[tw1980]

Why don't you guys trade tci's and see what happens.If that doesn't work try a coil or pick up assy.That's a quick way to eliminate things.I'm thinking we may start seeing some weird problems with these ignition units.30 year old components have a tendency to change value and become leaky.They're not to complicated I think we could probably come up with a new design but it would be a lot easier if someone knew of a schematic somewhere.

Terry

[dbeardslee]

Terry - that's what I was thinking. I've got a friend looking for one, unfortunately the 4R0's are harder to come by than the 2H7's. If he's got one then it's just 2 bolts and 2 electrical connectors and we'll know. DGXSER has an '81, but I understand his reluctance to try his TCI on this machine - the bad luck might seep out of Varnae's machine and right into his . The tech tips has a thread about building your own TCI. Unfortunately it say's it's for the 78-80 ignitions, so that kind of rules out the 4R0. At this point I'd like to get my hands on the real McCoy anyway, as I wouldn't want to introduce any unknown variables into what's already a tricky diagnosis.

[Ivan]

If nothing else, these guys can make an new TCI. The guys at Venture rider said they work pretty good and are programmable via a laptop for different advance curves. I think it was about $230 for one?

IngiTech

[dbeardslee]

Ivan - Czech Republic? How do you find this stuff?!! Pretty cool though, for a new TCI at that price - it's less than some used ones I've seen. I wonder if you have to pay any import fees on top of the $230. I didn't see the XS's listed, but they say they can build to suit. So who's going to be the guinea pig? Ivan, you don't speak Czechoslovakian by any chance...do you?

Riddle me this, batman: I seem to remember reading somewhere that the 4R0 TCI advances the ignition under normal operation. If that's true, doesn't it seem reasonable that a malfunctioning unit might advance the timing when it's not supposed to? I can't think of anything else that would account for the way-advanced spark at idle. Of all the things we've tested that's the only glaring inconsistency, and I keep thinking it's probably related to, or causing, the way it's running.

[Ivan]

I just saw it here:

Venture rider TCI thread

[Larrym]

Food for thought..

I don't have the 4RO unit as my bike is a 79 special which uses mechanical and vacuum advance. (My repair manual doesn't cover the later models which use a different advance system.)

In the absence of mechanical vacuum advance it appears that on the later models the TCIs do advance the timing. But "How do it Know when and how much to advance the timing?"

From what I can gather the vacuum from from #2 carb is connected to some form of transducer which then connects electrically to the TCI. In proper operation the "signal" from the transducer varies as the engine vacuum varies and therefore the TCI adjusts the timing to the proper degree.

Consider what would happen if the signal from the transducer did not vary and was stuck at the "level" where in normal operation, the TCI would be advancing the timing to the maximum level. Even if the vacuum line was disconnected and plugged the TCI would happily adjust the timing as if there were maximum vacuum. If the engine timing were to be set with this erroneous "advanced vacuum" signal then the TCI would be unable to advance the timing during engine operation so that the engine could run at top RPM.


Test this transducer. Possible that it may be just a simple "resistor" and that you can take a resistance reading with no vacuum applied and then apply vacuum for a second resistance reading. There should be some difference between the two. (Maybe your repair manual has the exact test/resistance readings...)


Also, I don't know if the 4RO unit uses centrifugal advance either. In centrifugal advance the timing is advanced as the RPM increases. If there is no mechanical centrifugal advance then the TCI does the advance by sensing the RPMs through the pick-ups for the cylinders or maybe through a separate signal from the tachometer? If this signal was missing then the TCI wouldn't advance the timing as the engine RPM increases.


Just spitballing here...

[DGXSER]

Larry,

Alot of good food for thought there!! The 4RO does not use any mech/centrifugal advance, it IS built into the TCI.

The other change over from the 2H7 system is that the 81 series does not use a ballast resister. It does use 3 ohm coils instead of 1.5 ohm coils.

So, not that this was specifically suggested, it would be alot to switch it all out. I believe running it with the wrong coils could fry the TCI. I think the timing issue will cause some poor running perhaps, but not the difference we are seeing.

I keep trying to figure out, why the timing shows to be so far off. Yet, when "correcting" it by rotating the plate, the closer we got to the correct timing, the worse the thing ran!!

Is it possible that the place that the rotor attaches to is bent?? or twisted??

[Larrym]

Mia?

so there is no vacuum line connected to a transducer somewhere?

[dbeardslee]

From what I've read I believe it determines the advance by rpms. Larry's probably right about it reading it from the pickup coils, but I don't know for sure. My point about the potential for a malfunctioning TCI to advance the timing is this - if it's malfunctioning at idle (advanced too far), maybe it's doing some other stuff it isn't supposed to as well - like cutting out at 4.5k. I'd sure like to try another TCI just to see. I heard back from my friend, and unfortunately he doesn't have a 4R0 in his parts cache. Varnae was all set to buy one the other day when we were discussing the situation, but I hesitate to tell him to go ahead without being certain that it is in fact causing the problem. Another recent thread suggested the regulator/rectifier as a potential cause for this type of condition. I don't know how that could cause the timing to advance, but who knows. I do have another regulator/rectifier that I know is good, so that at least would be easy to check. So, anybody got a 4R0 TCI they'd like to loan out for diagnostic purposes?

[XJOK2PLAY]

Doug,

FWIW...
Cody & I just made a discovery on mine yesterday concerning vacuum...

Where we had the pilot screws set... (we thought we had a good tune), according to the synch guages, we showed around 200 mm of vac.
But we were having all kinds of idle issues...

After tuning more critically, we watched the vac readings climb to 300mm!
Re-synched... major difference!

Low vac readings can be caused by where the pilot screws are set, it seems.

Thought it might be a reason why your vac readings are lower on this particular bike... we couldn't believe the difference. JAT
Good luck,
Bob

[randy]

I still don't see how just advancing the timing could cause such a dramatic rise in idle speed. Advancing the timing should only result in spark sooner than TDC, which, at idle, full advance would make the motor run worse. I still think it's intake / fuel related. When the boost hose is removed from the sensor, more air is available at carb 2 so i think that has something to do with the problem and it only appears to be caused by the boost sensor. that's my theory and i 'm sticking to it!

One more thing...the boost sensor's job is to RETARD the timing during high load / low vacuum conditions. One more variable for you !