Weird idle on my '90 (video)

[BluRoad]

So. I've had this 1990 VFR project for a few years, always seems to get put on the back burner, mainly because I've never been quite been able to get it to run just right. The problem: the bike starts instantly, warms normally, but after the choke is off it slowly begins to idle slower and rougher, with number two cylinder spiking the vacuum gauge (a cylinder miss with a mild backfire) and then eventually dying. It will fire right back up but the problem remains. It will run at higher RPM with no issue, so it has plenty of fuel.

 

What I've checked: The bike came to me as a non-running project. (I should mention that have lots of experience with the similar previous generation '86-87 VFRs, including carbs) I did all the usual stuff before firing the bike; valve adjustment, carb cleaning, fuel lines, filter, compression check (123, 153, 130, 142, note that #2 is the highest) etc. It fired right up and I sync'd the carbs. It has a K&N air filter and homemade shorty muffler modified from the original muffler. The emissions stuff has been removed (not a CA model) with the PAIR ports plugged and the three vent tubes exiting the carb set left open. When it just wouldn't idle correctly I removed and disassembled the carbs, checked them over, rechecked that the cams were installed to spec, swapped the #2 & 4 coils, then swapped the #2 & 4 coil wires, checked timing, swapped the (new) spark plugs. Plugging the large carb vent tube kills the engine. Plugging the two smaller vent tubes give no change. None of this helps.

 

I can't determine if it's a fuel, ignition or mechanical problem, though by swapping the #2 & 4 coils and wires, I seem to have eliminated ignition as the source (the miss stays with #2). Note that the vacuum rises and falls together on all cylinders. I'm not touching the throttle and the engine died on its own.

 

Any ideas?

 

YouTube link:

 

[5thave]

Compression check OK on that cylinder? Are the carb boots between the carb and the head dried/hardened and possibly leaking air? 

[BluRoad]

Compression is good, see text. Carb boots are new and soft.

[mobikie]

Because it affects all cylinders it makes me think it's a fuel problem. This is supported by the fact that it apparently runs okay when the choke is on.

[otmaximus]

When I removed the PAIR from one of my bikes I had a similar, though more pronounced problem when I missed one hose that needed to be blocked off.

Maybe there is a leak at one of the hoses/plugs.

[BluRoad]

Well, on the '86-87 there's a small port at each carb that needs plugging, but the '90 doesn't have these, instead there's ports linked by plastic tubes, like the carb vent (and fuel delivery) tubes. As I mentioned, plugging those makes no difference.

[Mohawk]

Does it run OK on choke ? If yes then its likely some crud in the idle mixture circuit. I have another bike with carbs that sat for 1 1/2 years because my leg was crock, then started fine idled for an hour, died at idle on first run, refused to restart, then starts after a carb clean, but shortly there after dies again. Waiting for another clean at the moment, but sounds like a similar issue to me.

[BluRoad]

Garage update:

1. When any of the air vent tubes are blocked off the engine dies.

2. Adjusting any of the pilot screws does nothing to the running of the engine, even running them completely in.

3. The popping on #2 is a backfire through the carb, as it pops off the vacuum hose.

 

JOE

[slovcan]

7 minutes ago, BluRoad said:

Garage update:

1. When any of the air vent tubes are blocked off the engine dies.

2. Adjusting any of the pilot screws does nothing to the running of the engine, even running them completely in.

3. The popping on #2 is a backfire through the carb, as it pops off the vacuum hose.

 

JOE

 

I would say if the pilot screws don't have any effect in any position, the carbs will need to come off again for a good ultrasonic cleaning.

[mobikie]

Backfiring is normally indicative of running lean.

[taglicious]

i was told by viffer93 couple years ago, to get a Factory Pro jet kit. i still haven't done it, but i have had this issue. your electronics grounded good? reg/rect, stator? I sent you a pm with my number. Use it and I'll be in touch as soon as I get home. 

 

Tag

[slovcan]

OK, I've thought about it a bit more and checked my manual. 1) The compression numbers you gave us are weird - too low (spec is 199 psi @ 400 RPM - anybody else get close to this?) and the spread is too big (10% is usually considered the max spread). 2) Did you adjust your valves BEFORE checking compression and syncing the carbs? 3) When you synced the carbs did you use #2 as the control (base) carb? 4) Double-check the whole emission/PAIR thing (hoses, vacuum ports/plugs, vents, etc.). There MUST be someone here who has successfully removed/bypassed/blocked this stuff. It might be different than your experience with your '86 (my experience with this is zero), so maybe go over your steps with them.

 

These are kind of back to basics, but can be overlooked sometimes.

 

Cheers,

Glenn

[BluRoad]

Thanks for the input, Glenn.

1. The compression check is necessarily done on an engine which has sat for X number of years. Marvel Mystery Oil in cylinders, valves adjusted, cold engine, carbs off. So I don't expect nice accurate readings under these conditions — it's just a way to verify there's not a major internal problem prior to firing up the engine. The front two cylinders are tough to compression check when warm because of the radiator, etc in the way and I want a compression check before initial startup. As a matter of interest, in doing compression checks on 20 second & third-gen VFRs I've never found one in excess of 165 psi (warm or cold), despite what the manual says. Most are near 150 psi.

2. Yes

3. Yes, as the video shows, the carbs sync'd nicely.

4. Emission stuff is actually pretty straightforward; block the injector ports and vent the vent ports to atmosphere. Toss a whole bunch of hose and hardware.

 

I'm on the hunt for another set of carbs to eliminate or confirm the carburetion as an issue. I've also procured a second ECU to do the same with the ignition.

 

JOE

V4 Dreams (.com)

[gswanson]

Backfiring through the carb would make me think it is either ignition timing, a sticking or non closing valve or possibly carbon build-up that heats up and causes pre-detonation.  Can you do a leak down check?  That will show more than a compression test alone.  Try and do cold and then with the engine warmed.  Seems to be heat related somehow.

[BluRoad]

I would love to do a leak-down test, but I don't have the equipment. I know the value in the test, used it regularly on aircraft when I was into that. That would surely eliminate an internal mechanical issue.

 

The valves, as visible through the intake tracts, look clean, straight and operating normally. The cams & lobes look great, and the cam timing triple-checked. My thinking is that if it were a mechanical issue it would be evident all the time — hot cold, choke on or off, etc. It's not. It's intermittent. What's confusing to me is that all the cylinders decrease & increase in unison — a single miss wouldn't kill the engine — it will run on three or even two cylinders. For all these reasons I'm still leaning towards carbs. We'll see.

 

Joe in IL