1977 850-T3 delayed idle issues.

[n3303j]

So I bought this critter with 20K miles on the clock. Added Ghilardoni cylinders and all rubber bits (and a bunch of other little bits that kept it running reliably). Then I rode it about 100K miles further.

About 10K miles ago the bike started having problems returning to idle once the engine was past 3,500 RPM. Engine would take half a minute to come down to an 1,100 RPM idle. That meant no engine braking. If I slowed the engine to 1,500 RPM with the brakes then disengaged the clutch the engine would drop right to normal RPM. If I pulled the clutch and closed the throttle the engine would return to idle in about 30 seconds.

Went through the carbs but things didn't improve. Timing and valve clearances are in spec.

Was burning oil (100K miles on rings & valves) so I just replaced the rings and had the heads done by Randy Long. That did not resolve the issue.

Engine accelerates smooth and pulls strong at all speeds. It just won't drop quickly to idle when the throttle is closed unless I set the idle stops to the point that the engine will not idle.

I'm guessing that I'm having an issue with the mechanical advance. It's almost like the weights are hanging on the return stroke. I did disassemble, deburr, clean, lubricate and reassemble the whole advance assembly including new return springs That made no change. It does have worn spots but they don't appear to hang when actuated by hand.

So I'm looking for ideas for a direction to remedy this issue. This is my favorite machine and I do want to ride it another 100,000 miles WITH engine braking.

Help! Thanks, Ron

[Dirk_S]

You don’t think it’s an air leak/vacuum issue?

[n3303j]

Just reassembled whole top end with all new gaskets and seals. It seems tight. Suppose I'll check junction between carb & manifold. Have never been able to get those items apart. They were frozen together since purchase. But I'll spray around the areas and look for an idle change.

[wirespokes]

How long since replacing the rings?

Have the heads been re-torqued?

It sure sounds like air leaks to me.

I wouldn't worry about the carb/intake manifold junctions. If they won't budge, I can't imagine them being the problem.

[n3303j]

Rings & valves 80 miles and two heat cycles ago.
They will be reset before next run.
I usually re-torque 100, 250, 500, 1,000 and 1,500.

But the problem was an issue that began around 10,000 miles ago (110,000 miles on distributor and whole bottom end)

[acogoff]

  Double intake gasket between head and manifold. At idle spray it with something- WD, Carb Cleaner, etc. worth a try. leaked on mine gave similar symptoms.

[n3303j]

  Double intake gasket between head and manifold. At idle spray it with something- WD, Carb Cleaner, etc. worth a try. leaked on mine gave similar symptoms.

Gasket replaced 80 miles ago. Symptoms did not change.

Would aged carburetor slides or a worn interface between carb & slide cause an issue? Carbs are 48 years & 120,000 miles old.

[moto]

Gasket replaced 80 miles ago. Symptoms did not change.

Would aged carburetor slides or a worn interface between carb & slide cause an issue? Carbs are 48 years & 120,000 miles old.

That seems possible. However, the slide is flat, as is the face on which it bears, so inspecting it could be helpful. I believe you can sand the slide flat, and then use it to sand the face. Dave Richardson noted the inherently long life of the flat slide design, compared to round slides, due to the flat surfaces maintaining a better seal. If the surfaces look smooth and flat, I doubt that this is the problem.

[Gliderjohn]

Have you checked for sure that your cables are not hanging up anywhere? I have had the problem in the past with the same symptoms.
GliderJohn

[pehayes]

Too sharp of curvature on throttle cable routing?
Aged, weakened throttle return springs (usually people complain they are too strong and hard on the wrist)?
Can you borrow a pair of carbs from someone who has a bike in disassembly???

Patrick Hayes
Fremont CA

[n3303j]

Cables are fine. Work free and easy. Release throttle and I can hear the slides hit the stop screw. Enough cable freeplay so I can slide the jacket back & forth about 2 millimeters.

If I brake without clutch then pull clutch in about 1,500 RPM the engine immediately drops to proper idle. Wouldn't do that with hung cable.

If I set idle stop way too low engine will come down normally but will die if I don't get on a bit of throttle at idle RPM. So I guess I'm choking it into a deceleration.

[Dukedesmo]

To try and mitigate the heavy throttle action on my LM2, I bought lighter carb springs. I actually bought both the 'light' and 'medium' Dellorto springs.


First, I fitted the light springs. All seemed good as the carbs snapped shut fine with the engine not running but once running it would hang at a higher RPM for a while. I fitted the medium springs and all is good and there's not much difference in weight between them and the light - still a heavy throttle but I've got used to it.

[Tkelly]

Same thing happened with my t3 when I started it with the choke on one carb and forgot to turn it off.

[wirespokes]

There are rubber seals on the choke slides. I suppose one or both could be defective. What kind of fuel mileage are you getting?

It's also possible the throttle springs have gotten weak - that's another possibility.

[n3303j]

Enrichers both closed, but been awhile since I've examined their seals.

But what's throwing me off is that I have a solid 1,100 RPM idle when the engine settles down.

I'm also running the lighter slide springs. But have been running them long before the issue started. Suppose I'll break out the originals and see if that makes a difference.

I was really expecting some comments on worn ignition advance units. But evidently that's not a frequent problem.

[Dirk_S]

When I took the carbs apart on the BMW R80/7 for the first time, I accidentally swapped the enrichener plates on the side of the Bings. That was hanging up my idle at times. But I highly doubt you’re as careless as I…ever.

[n3303j]

When I took the carbs apart on the BMW R80/7 for the first time, I accidentally swapped the enrichener plates on the side of the Bings. That was hanging up my idle at times. But I highly doubt you’re as careless as I…ever.

I remember that with the Bings on my R100RT. The forum spent a bunch of time discussing the fact that the parts could be assembled the right way or the OTHER way. Yup, I've done it both ways too.

[Tom H]

You might be a little lean on your idle mixture screws. Had that happen. Also, make sure your advance mechanism is working smoothly, a quick shot of lube on the pivots may help. Also, make sure your timing is correct. The "light" carb springs "supposedly" can sometimes cause the slides to hang due to vacuum pressure.

Hope this helps,
Tom

[wirespokes]

Richardson (guzziology) says, as I recall, that lighter springs on the VHDs isn't recommended due to the necessity of seating the needles into the accelerator pumps. That's about the simplest change to see if that doesn't fix it - re-install the stock springs. And it does make sense that it's the source of your ills.

[jacksonracingcomau]

En 3
Well done, we all now know what is not the problem, elimination process.
I would now
1/ Change slides (right for left and v-v), if already done, buy new, you can only do this once !
2/Set idle accurately with gauges/manometer
3/Conform synchronous actuation with above
4/ Let owner ride/drink beer/go for a ride myself

Pix of slides from underneath to explain to others, please

[wirespokes]

Wouldn't it be a kick in the head if all it was was weak throttle springs? Rebuilt the whole thing when all it really needed was a couple of springs. 

[n3303j]

Wouldn't it be a kick in the head if all it was was weak throttle springs? Rebuilt the whole thing when all it really needed was a couple of springs. 

It's not that, because when it first started happening I removed the MG Cycle light springs and replaced them with the Moto Guzzi Carpal Tunnel specials. It changed nothing except for old person wrist agony.

Meanwhile I'm wrapped in a bunch of Honey-Dos so that bike is waiting patiently in my shop. I will get back to addressing addressing all the issues and will report success or failure.

[wirespokes]

Damn! I was sure that would be it!
At this point it sounds like you've been addressed all the suspects. I've heard that the carb top gaskets can leak air and cause problems, but doesn't make sense to me.
Perhaps it's time to source a second set of carbs and a distributor to verify it's not anything to do with them.

[n3303j]

Honey-Do's are done and I got a day with the T3.

I really liked the idea of swapping right/left slides. The slides wear to their mating surface and have a lot of contact area. Swapping them decreases the mating surface contact area and hence reduces friction.

I went a step further and did a light lapping on the face of the slides to reduce irregularities. It smoothed out the bare aluminum but hardly effected the dark anodized spots. Guess those are low spots on the slide. I also put a bit of 600 grit on the slide face and ran it down and up the slide chamber just enough to remove any major irregularities. Finally I finished assembly using the original Carpal Tunnel slide springs.

Did a quick adjust and then 10 miles to heat up everything. Returned to the shop and set final idle mixture, speed and synchronization.

Did a second 10 miles and I have repeatable engine braking and proper deceleration to around 1050 RPM. Life is good in T3 land. Now I need another 900 miles to settle all the new gaskets and bed in the rings. Then back to synthetic oil and long runs.

If it happens again down the road I guess it's a new pair of -40 slides with full hard coat on the working surface. Glad it isn't my advance unit's wear. I don't see any NOS advances for sale anywhere.

Thanks for the advice. Ron

[moto]

...
I went a step further and did a light lapping on the face of the slides to reduce irregularities. It smoothed out the bare aluminum but hardly effected the dark anodized spots. Guess those are low spots on the slide. I also put a bit of 600 grit on the slide face and ran it down and up the slide chamber just enough to remove any major irregularities. Finally I finished assembly using the original Carpal Tunnel slide springs.
...

Great report!

As it turned out, you changed two things to improve your bike. One was the slides, and the other was the return springs. So strictly speaking we don't know which one fixed the problem, I think.

Would you be game to swap back in the limp throttle return springs and see whether your problem is still fixed? If you did that and found that the problem was still fixed, we would have valuable new information that slides can be a serious problem for proper idle control down the road.

I only ask because swapping out the slides return springs one more time is so simple an experiment. Congratulations on fixing your problem.

[n3303j]

I'm too old and lazy and to swap back to the light springs then do all the confirmation trials.

But when the problem began last year I did revert to the original springs and the problem did not go away. So I'm pretty sure springs were not the answer. I did not swap or modify the finish on the slides at that time.

So the change that remedied the problem was creating reducing slide friction by reworking the slide surfaces and swapping left for right.

If I ever replace the slides or find someone to recoat the worn slide face I will reconsider the light springs. Right now my wear surfaces are two equally hard parts of aluminum. Better when the two surfaces are dissimilar in hardness.

PTFE impregnated anodize would be an ideal coating but probably minimum charges would exceed cost of two new slides. I think the new ones are hardcoat anodized. So I'll question a few plating shops and in the meantime ride the heck out of her. I really do like my red bike.

[moto]

I'm too old and lazy and to swap back to the light springs then do all the confirmation trials.

But when the problem began last year I did revert to the original springs and the problem did not go away. So I'm pretty sure springs were not the answer. I did not swap or modify the finish on the slides at that time.

So the change that remedied the problem was creating reducing slide friction by reworking the slide surfaces and swapping left for right.

If I ever replace the slides or find someone to recoat the worn slide face I will reconsider the light springs. Right now my wear surfaces are two equally hard parts of aluminum. Better when the two surfaces are dissimilar in hardness.

PTFE impregnated anodize would be an ideal coating but probably minimum charges would exceed cost of two new slides. I think the new ones are hardcoat anodized. So I'll question a few plating shops and in the meantime ride the heck out of her. I really do like my red bike.

I can buy all that. Sorry to suggest excessive work. I think I know how you feel.

[wirespokes]

Well done! Now you've got a new bike all because of worn slides. 

I really doubt that friction had anything to do with it. In fact, I'd think there was less contact before, which would equate to less friction. But that's just guessing and it's possible there was no change in the amount of friction.  I'm thinking the difference is a better seal - less air leak past the slide.

[n3303j]

Well done! Now you've got a new bike all because of worn slides. 

I really doubt that friction had anything to do with it. In fact, I'd think there was less contact before, which would equate to less friction. But that's just guessing and it's possible there was no change in the amount of friction.  I'm thinking the difference is a better seal - less air leak past the slide.

New rings and valves (after 100K miles) helped with the "new bike". Leakdown hissed at exhaust and oil fill holes.

But if the issue was air leakage past the slide then I should probably consider facing the surface of the carb and replacing the slides with new ones with their factory fresh hardcoat.

I sort of can believe air leakage. V11 achieved idle with throttle plate at 2.5° or so. That's not a lot of air passage.

[Tkelly]

It is a great feeling to get on your old bike and have it run just like it did back in the day.It is different from a modern bike where I sort of take it for granted that it will run smoothly every time.My theory is that there was a great jump in modernity in the bikes built in the late 60 s and 70s,everything since is fluff.