Author Topic: engine self accelerates (Read 2992 times)

Sparked · « **on:** June 10, 2012, 08:36:24 PM »

Ok, I have about 30 miles so far on the bike since swapping in a new-to-me engine. I have changed the oil, set the points, valves, and cleaned the carbs (same carbs as where on the last engine). The donor parts pike hat about 10,000 miles on it and all I have done was take off the valve cover to check camshaft wear.

There are a few symptoms that have crept up and I am wondering what I should be looking at for causes?

-Gutless acceleration (especially below 5000 rpm)
-kick starts easy, electric start not so easy
-hard to shift when the bike isn't moving
-the last 5 miles the engine would accelerate by itself when sitting at a light. Had to slip the clutch to get the revs to come back down.
-neutral light stops working when the engine warms up.

Thanks in advance, I am looking forward to getting these gremlins sorted out of this engine and spending some good seat time on my little 350f.

dave500 · « **Reply #1 on:** June 10, 2012, 08:50:06 PM »

check for vacuum leaks around the inlet manifold,,try adjusting the carb idle screws,,it sounds like it might be going lean at idle to give that rpm increase?

flybox1 · « **Reply #2 on:** June 10, 2012, 09:09:57 PM »

Quote from: dave500 on June 10, 2012, 08:50:06 PM

check for vacuum leaks around the inlet manifold,,try adjusting the carb idle screws,,it sounds like it might be going lean at idle to give that rpm increase?

+1

Sparked · « **Reply #3 on:** June 10, 2012, 09:19:07 PM »

Idle screws are all set 3/4 turns. Turning the screws out will richen the mixture? Say 1/4 turn at a time?
thanks for the quick response.

dave500 · « **Reply #4 on:** June 10, 2012, 09:24:20 PM »

any of those settings for number of turns out on carbs for any engine are ball park really,,they may have been ok when the bike was factory new but they change as an engine wears,if it needs more richness then richen it a little at a time.,turning out will lean the mixture,,,umm?you may be too rich now with an air leak?go over the igniton again and be sure the advancer is free to turn.

Sparked · « **Reply #5 on:** June 10, 2012, 09:28:47 PM »

Turn out = lean
Turn in = rich

I will play with the idle screws tomorrow after work. Should I make sure the engine is warm before adjusting the idle?

dave500 · « **Reply #6 on:** June 10, 2012, 09:33:12 PM »

yep,do it warm,,go for a ride and take a long screw driver that will reach the inside carbs,dump a strong magnet on the bars and stick the screw driver on it.

HondaMan · « **Reply #7 on:** June 10, 2012, 10:04:09 PM »

Quote from: Sparked on June 10, 2012, 09:19:07 PM

Idle screws are all set 3/4 turns. Turning the screws out will richen the mixture? Say 1/4 turn at a time?
thanks for the quick response.

Go 1/8 turn or less at a time. Stay within 3/4 turn of the stock [number of turns] in any case. Most likely, the O-rings between the head and the intake tract are old and leaky, if not the hoses to the carbs (or the clamps).

Sparked · « **Reply #8 on:** June 11, 2012, 07:03:22 PM »

Throttle screws were set at 1.5 turns as per manual. I have turned them in 1/8th turn and tightened up all the carb boot clamps. I left it running in the yard until it was warm and started to accelerate before making the adjustments. Going to take it out for a ride and see if it runs better.

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Sparked · « **Reply #9 on:** June 11, 2012, 09:32:07 PM »

Turned the screws in a total of 1/4 turn so they are now set at 1-1/4 turns out. It is running smoother and no longer self accelerates.
It stumbles really bad and bearly accelerates between 4000-5500rpm.

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Spanner 1 · « **Reply #10 on:** June 11, 2012, 09:47:08 PM »

And the bike is ??....... have you vacuum synced the carbs ?...are your float levels correct ? and are the bowls filling to the correct level ? Is rust/gunk from the tank blocking the petcock channels ( pretty common on a lot of bikes actually, and everything else blamed first

).... just a few questions

Sparked · « **Reply #11 on:** June 11, 2012, 10:48:59 PM »

Quote from: Spanner 1 on June 11, 2012, 09:47:08 PM

And the bike is ??....... have you vacuum synced the carbs ?...are your float levels correct ? and are the bowls filling to the correct level ? Is rust/gunk from the tank blocking the petcock channels ( pretty common on a lot of bikes actually, and everything else blamed first ).... just a few questions

1972 cb350f stock w/400f exhaust.
No I have not vacuum synced the carbs.
Float level was correct when i rebuilt the carbs last time, didn't think they would change.
Not sure to what level the bowls fill too. How do i check that?
The tank is pretty clean, and there is no sediment in the inline fuel filter.

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Rgconner · « **Reply #12 on:** June 12, 2012, 07:43:21 AM »

Have you sprayed carb cleaner around the intake and head side boots when it is idling fast?

A leak will draw it in and you get a sudden changing in RPM.

A non-leaking system will not draw any in and there will be no change.

Oh, stock intake/airfilter?

Sparked · « **Reply #13 on:** June 12, 2012, 07:54:59 AM »

I will try the carb cleaner.
stock paper filter and airbox.
I believe stock jets. Should have written down the numbers when i had the carbs apart.
The carbs & exhaust are the same as before. The only thing that has changed is the engine.

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TwoTired · « **Reply #14 on:** June 12, 2012, 08:41:51 AM »

Quote from: Sparked on June 11, 2012, 10:48:59 PM

No I have not vacuum synced the carbs.

Why not?
You really should do this. ...Or, at least check vacuum balance.

Sparked · « **Reply #15 on:** June 12, 2012, 10:41:06 AM »

The carbs worked great on the last engine. I thought it would be more of a plug-and-play to get up and running again. But different engines must have different requirements so what was setup and worked on one, won't always work on the next.

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TwoTired · « **Reply #16 on:** June 12, 2012, 01:02:16 PM »

The vacuum is generated by the engine. The carbs can only balance with what is there among the cylinders. If the two engines generate different vacuum levels, then carbs will not be sync'd after the swap.

If the vacuum levels are the same between engines, I'd have to wonder why the swap was necessary?

Anyway, I think it should be verified, before changing other parameters. It's on my tune up schedule.

Sparked · « **Reply #17 on:** June 12, 2012, 01:11:59 PM »

TT
That makes perfect sense to me now. Will sync the carbs.

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Sparked · « **Reply #18 on:** July 06, 2012, 10:49:47 AM »

Got around to re syncing the carbs yesterday. It is running much better now. No high idle, starts even easier and more bottom end power. I am going to put new plugs in this weekend and do a plug test.

There is still a hesitation in the rpm band around 4200 rpm to figure out though.

lucky · « **Reply #19 on:** July 06, 2012, 11:49:50 AM »

For these transmissions to shift the engine should be running and if the back wheel is turning a little the shift dogs can line up.

if you pounded the shift lever while it was sitting, and not running, then you probably bent or broke the tranmission. It is not a car.

Sparked · « **Reply #20 on:** July 06, 2012, 01:09:12 PM »

Quote from: lucky on July 06, 2012, 11:49:50 AM

For these transmissions to shift the engine should be running and if the back wheel is turning a little the shift dogs can line up.

if you pounded the shift lever while it was sitting, and not running, then you probably bent or broke the tranmission. It is not a car.

Thanks Lucky.
Is there a way to diagnose the transmission without splicing the cases? If I have to that's not too big of an issue. I have all the internals from the last engine that are in good shape. If there is internal damage then I can chalk one up for the PO.

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Author Topic: engine self accelerates (Read 2992 times)