Author Topic: Jerking on Acceleration (Read 5157 times)

mcinturff · « **on:** May 03, 2015, 05:10:56 PM »

I just put a new chain and sprockets kit, among other things, on my '74 750 after sitting for 25 years. Initially I thought that the sprocket was skipping a tooth on acceleration because they were both severely worn and the bike would jerk if I opened the throttle too much. The only thing left is the clutch and the primary chain. I already took the clutch assembly apart, checked and cleaned everything, and put it back together after checking to see if all parts were within tolerance (they were). Even after all this work, the bike still jerks under loading from acceleration. Any ideas? I'm certain this is not a tuning issue, the engine runs like a Swiss watch. Perhaps the clutch plates are warped, or the cork material is pretty much toast from hardening.

seanbarney41 · « **Reply #1 on:** May 03, 2015, 06:08:31 PM »

does the bike do it in all gears? I had a bike that I thought had clutch probs but it would only misbehave in 2nd...turned out something was wrong with 2nd gear so that it would pop out and fall/bounce back in...I have not yet tried to fix it.

mcinturff · « **Reply #2 on:** May 03, 2015, 06:36:34 PM »

Yep, 1st through 5th

flybox1 · « **Reply #3 on:** May 03, 2015, 07:10:46 PM »

Time for a total carb clean...

mkoski · « **Reply #4 on:** May 03, 2015, 08:14:22 PM »

I'm going to disagree with everyone because I think I know what you are talking about. Here's what I think: tighten your chain, bring it to a small amount of slack, almost zero so it has some tension. My jerking issue was the exact same, nothing to do with engine.

Let me know if this works!

mcinturff · « **Reply #5 on:** May 03, 2015, 09:05:30 PM »

Quote from: flybox1 on May 03, 2015, 07:10:46 PM

Time for a total carb clean...

Carbs are rebuilt already, cleaner than an operating room when they went together. Bike runs great. Slipping/jerking seems to be related to amount of torque produced, not how quickly the throttle is opened.

flybox1 · « **Reply #6 on:** May 03, 2015, 09:15:42 PM »

Glad they're clean...but is the mixture right? What do your plugs look like? Have you done the clear tube test to prove the fuel level in your bowls is correct?

evanphi · « **Reply #7 on:** May 04, 2015, 04:39:25 AM »

Cush drive?

mkoski · « **Reply #8 on:** May 04, 2015, 07:14:32 AM »

Mcinturf,

I think we need a little clarification here.

1. Does the bike just jerk once when you open the throttle to accelerate and then no more if you keep the bike accelerating?
If so then I still think its a loose chain, or, as Evanphi mentioned, could be cush drive. This will be caused by the engine taking up the chain slack (or the gap in between your cush drive components in the hub). If you are getting a hard jerk only once when accelerating, right when you open it, then thats it.

2. During steady acceleration is it jerking? If so then look to Flybox, as this jerking will be caused from some kind of power delivery issue in the engine/carbs.

Thanks,

MJK

jodangler · « **Reply #9 on:** May 04, 2015, 03:23:50 PM »

I'm sure you already checked your timing/gaps but I had some thing like that happen to me yesterday and once I took the timing plate cover off, one bolt fell out and the others were loose on the timing plate and made everything gerk around on all gears, after fixing it and applying locktight she's I'm back on the rode cruisin...

mcinturff · « **Reply #10 on:** May 04, 2015, 08:53:11 PM »

Quote from: mkoski on May 04, 2015, 07:14:32 AM

Mcinturf,

I think we need a little clarification here.

1. Does the bike just jerk once when you open the throttle to accelerate and then no more if you keep the bike accelerating?
If so then I still think its a loose chain, or, as Evanphi mentioned, could be cush drive. This will be caused by the engine taking up the chain slack (or the gap in between your cush drive components in the hub). If you are getting a hard jerk only once when accelerating, right when you open it, then thats it.

2. During steady acceleration is it jerking? If so then look to Flybox, as this jerking will be caused from some kind of power delivery issue in the engine/carbs.

Thanks,

MJK

Above 3k rpm it will suddenly jolt like it grabbed something (almost like skipping a tooth somewhere) if you open the throttle to accelerate moderately quick. I haven't tried to continue to accelerate after it jerks once out of fear of damaging something. The drivechain is adjusted to take any excess slack out.

On another, possibly related, note the clutch doesn't begin engage until the lever is almost completely released. I have adjusted the cable properly, and the cable itself is in relatively good condition. I wonder if the cork plates have become "petrified" or something due to sitting all that time. Either way, I have new ones coming with a batch of other parts for the bike.

mcinturff · « **Reply #11 on:** May 04, 2015, 08:55:11 PM »

Also, I'm working on trying to get ahold of some sync gauges just to be sure, anyone know where I can get a reasonably priced set?

evanphi · « **Reply #12 on:** May 05, 2015, 05:02:05 AM »

Quote from: mcinturff on May 04, 2015, 08:53:11 PM

Above 3k rpm it will suddenly jolt like it grabbed something (almost like skipping a tooth somewhere) if you open the throttle to accelerate moderately quick. I haven't tried to continue to accelerate after it jerks once out of fear of damaging something. The drivechain is adjusted to take any excess slack out.

On another, possibly related, note the clutch doesn't begin engage until the lever is almost completely released. I have adjusted the cable properly, and the cable itself is in relatively good condition. I wonder if the cork plates have become "petrified" or something due to sitting all that time. Either way, I have new ones coming with a batch of other parts for the bike.

Sounds like a mixture issue. 3K is pretty low on the revs... if you're slowly accelerating to 3K and then snapping the throttle open and your mixture isn't just perfect, you're going to get a surging/miss. Stock airbox? Pods? Do a plug chop and report back here with results.

Clutch engaging way out is normal.

chewbacca5000 · « **Reply #13 on:** May 05, 2015, 05:13:46 AM »

A leaky petcock can cause problems as well. Those fibre disk that secure the petcock as well as the rubber selector really need to be replaced.

mcinturff · « **Reply #14 on:** May 25, 2015, 11:04:00 AM »

I finally got a new clutch on the bike. I can tell that the clutch was part of the problem. Now the bike only jerks in 2nd gear, if you get on the throttle it will jump out of second. I'm going to cross my fingers and hope its in the shift linkage, but the most likely thing is bad dogs on 2nd gear.

mcinturff · « **Reply #15 on:** June 10, 2015, 08:06:44 AM »

Here's the culprit

70CB750 · « **Reply #16 on:** June 10, 2015, 08:19:16 AM »

Quote from: mcinturff on May 25, 2015, 11:04:00 AM

I finally got a new clutch on the bike. I can tell that the clutch was part of the problem. Now the bike only jerks in 2nd gear, if you get on the throttle it will jump out of second. I'm going to cross my fingers and hope its in the shift linkage, but the most likely thing is bad dogs on 2nd gear.

I have that. It started jerking first only if I downshifted to 2nd for a turn, than it would jerk on acceleration. Quite quickly it got worse and now I dont use the 2nd at all, just speedshift to three going up and to neutral going down.

The plan is to rebuild the K0 in the winter,but the K3F has to be reliably on the road first.

mcinturff · « **Reply #17 on:** June 10, 2015, 08:24:26 AM »

I got a good used trans off eBay for $50. Going to try and put it back together this weekend.

Don R · « **Reply #18 on:** June 10, 2015, 08:44:45 AM »

Good luck, inspect it all well, you already know what to look for. Just for giggles if you haven't already check the rear brake, I've seen the friction material come loose. That's what I was thinking reading the start of the thread.

Duke McDukiedook · « **Reply #19 on:** June 10, 2015, 08:45:18 AM »

Quote from: flybox1 on May 03, 2015, 07:10:46 PM

Time for a total carb clean...

CLEAN. ZE CRABS.

mcinturff · « **Reply #20 on:** June 10, 2015, 08:49:56 AM »

Quote from: DukieDukenDuke on June 10, 2015, 08:45:18 AM

Quote from: flybox1 on May 03, 2015, 07:10:46 PM
Time for a total carb clean...

CLEAN. ZE CRABS.

Did you even read the replies? especially mine with 2nd gear removed.

mcinturff · « **Reply #21 on:** June 10, 2015, 08:52:25 AM »

I don't know how well everyone can see it in the picture, but 2nd gear dogs are TOAST

Don R · « **Reply #22 on:** June 10, 2015, 08:55:24 AM »

They are toasty, second is the usual suspect.

flybox1 · « **Reply #23 on:** June 10, 2015, 08:56:23 AM »

Yeah, read the replies...but those dogs will either slip, or they wont.
Their wear doesnt explain your jerking on acceleration.
if your carbs are as clean as you say they are, then your mixture is not right.

flybox1 · « **Reply #24 on:** June 10, 2015, 09:00:22 AM »

Glad you found a new trans. I had to do it too. Its a PITA to remove the engine and such, but its a pretty straightforward job.
It'll fix the slipping out of second issue, but I still have a hunch you'll have the jerking on accel to deal with....

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Author Topic: Jerking on Acceleration (Read 5157 times)