xj750r pulsates at 3500 rpm

You guys are going to think I'm lying - but this has been bugging me for months so I thought I'd throw it out and see if anybody can help. I've got a 1982 XJ750R with 23K on it. I've owned the bike for about a year. I rebuilt the carbs - the right way - even separated the bodies and replaced the seals. Also bought a ColorTune (and set the idle jets accordingly) and sync'ed the carbs. Wet set the floats - did eveything right. I also set the valves correctly.

However, I have an odd symptom - when I run the engine at a constant 3500 RPM, the engine pulsates (for lack of a better word). It's like the throttle is being hit and released several times per second - so the bike jerks a little - pulsates.... But this only happens at 3500 RPM. If I slow the engine to 3400, no problem. If I speed up to 3600 - no problem. So in a debugging effort, I removed the carbs from my other XJ750R (blue) and put it on this bike (red). The same problem - the red bike pulsates at 3500 RPM. I took the newly rebuilt carb and put it on my other bike (blue) - and the blue bike ran great. So this indicates to me that it is not a carb problem - that it is something else.

I thought about the YICS ports so I removed the outside bolts on each side of the engine and everything looked clean - I could see straight through. Plugs are new and look fine.

Any ideas? 'cuz I'm out of them.

 

Is this happening on the road, or on the center stand?

 

K-moe - it happens on the road. Doesn't matter what gear - it only happens when the engine is at 3500 RPM.

I have not tried it on the center stand - but I will just to get more data points.

 

Strange these types of engine issues can be hard to track down , could be an internal problem with the engine, cam chain guide is possible . You stated you did the valve shims you might want to recheck and check to see if the chain has tension , there might not be a noise but a vibration. One other thing inspect the motor mounts , for security .

 

K-moe... I checked the behavior with the bike on the center stand... I slowly rolled on the throttle until the rpms hit 3500. The engine seemed a little tired. Then when it hit 3600 the engine came alive and rpms JUMPED to about 5000 with no additional throttle roll. So now I'm thinking it might be a weak coil or a spark plug wire that has too much resistance and once the engine gets up to speed, there is enough voltage to overcome the resistance...? Any other thoughts?

 

With regards to the YICS port, you may be able to see thru from one side to the other....but there is 4 holes in there that need to to be clear as well.......Chacal sells a cleanout tool specifically for these ports........

however I am sure this is not related to your problem but wanted you to be aware of these 4 holes............

 

That's a really odd behavior. The engine isn't supposed to do that until it hits 5,500 RPM.
Do you have an external tach that you can use to verify that your tach is reading correctly.

If the pulsing is only occuring on the road, then I'd look at the wheel and tire balance, and inspect the tires closely for odd wear patterns.
That's just to rule that out before moving on to other possabilities.

 

this makes me think carbs or TCI advance curve or plugs. are all the pipes equally hot at startup?

could it be missing at 3400, trying at 3500 and start firing at 3600, that will make the rpm's will shoot up every time

 

I'm sure the tires are balanced - they are new and ride well. I'm sure the tach is working correctly. It shows about 6000 RPM at 80-ish MPH (same as my blue xj750). I now think the pulsating at 3500 RPM is because at that engine speed, it's trying to jump to 5000 but since it's in gear, it can't, so it slows a little then tries to jump but can't - etc. I'm going to start checking the plugs, wires, coils on Saturday when I get some time. In the meantime, I took a video - here the link:


In this video I am slowly rolling on the throttle - you can see at about 3500 or 3600 the engine speed jumps. However I don't change the throttle roll at all.
Question t0 K-moe.. you said it wasn't supposed to do that until 5500 RPM... what does that maen?

 

At 5500 RPM the engine comes onto the cam and screams up to redline, almost before you know it. This is when you either start to grin, or poo your pants (depending on how experienced of a rider you are).


You have a misfire in the lower RPM range ( I can hear it).
I agree with Polock that it might be an ignition problem.
It might also be a fuel issue thats come up post-cleaning. Are you running an inline fuel filter?

Have you pulled the spark plugs to read them?

How it's acting makes me think that she's lean at idle and the misfire clears up once it transitions onto the mains (even though it should already be on the mains before the RPM that she clears up at).

 

What they said....missing, running on 3? until 3600, > then running perfectly.....

 

Originally I thought it was the carb jetting (as K-Moe recently suggested), however as I indicated in the first post - this same problem exists even if I swap carbs with my other bike.

This morning I checked all four cylinders for spark - I hooked up a timing light to each plug wire while the bike was running. Each cylinder seemed to have strong spark. (the timing light was flashing at the same rate on each cylinder). All cylinders seem to be working because the exhaust pipes all heated up at the same rate.

I pulled the plugs (see picture) number 1 cylinder (far left) looks red. I don't know it the plug is causing the problem or if the problem is turning the plug dark...I'll throw a new set of plugs in it tomorrow and see what happens.

 

Could be a defective plug, it happens, breaking down under compression, try swapping plugs over, see if the 'condition' follows the plug, or transfers to 'that' cylinder/plug'.....

 

The plug second from the left shows signs of that cylinder running dangerously lean (there is a ball of aluminum on the ground strap). The two to the right of that one look a bit lean as well.
The far left plug is about right, if not a tad rich.

What method are you using to set the idle mixture, and what jets are in the carbs?

Are your other carbs from the same model?

 

When I rebuilt the carbs I set them to factory specs for all jets. The carbs on both bikes are set up the same.

I put new plugs in today and rode for 20 miles. See attachment for the picture of those plugs.

I also have another video. You already saw the rpms jump on my previous video. However, when the rpms ate decreasing, they suddenly drop at 3500... drop by 1500... the same amount the JUMPED when the throttle was increasing... ??? I'm really confused now.

 

The power band ROLLS on around and above 5500 on up, but it's not a quick jump though.

I hear a good miss in the lower rpms. It almost sounds like it's not running on every cyl.

 

Missing, as above.......

 

Could it be your pickups/timing, or even TCI....advance gone haywire??

 

There's so many things that it COULD at this point.

1. It sounds like a carb issue
2. The plugs look terribly lean
3. It acts like a TCI issue
4. Etc....

The way it acts and sounds just before the jump seems to me that a likely scenario is this:

It's running lean, AND you have one cyl not firing. As you approach 3600rpm, that cyl starts trying to fire as it is now starting to get fuel. Once it gets it, it finally kicks in, til rpms come down enough to where it loses enough fuel.

That would indicate potential carb issue.

BUT- you did the right thing in swapping carbs around. The problem did not travel, so let's leave carbs out for now.

That seems to bring it to a mechanical or electrical issue. You said you checked with a timing light and all plugs appear to be firing steady and correctly. That SHOULD rule out an electrical issue.

That would SEEM to bring it to a mechanical issue. That leads me to suspect a valve/bent valve/compression issue.

You said you did the clearances.... what spec ranges did you use? What chart? What were clearances, and what did you change them to?

Also, can you do a compression test and tell us the results?

Dave

 

Dave,

I verified that intake valve clearance was 4 to 6 thousands of an inch and output was 6 to 8 thousands. I had to replace 2 shims if I recall correctly.

I just did a compression test. Throttle fully open and about 5 or 6 compression strokes as the engine was spinning - until the needle on the compression meter stopped moving. I let the bike sit over night. I did a dry test and then squirted a little oil in each cylinder for the second test. Results: dry- 100, 100, 100, 120
Wet- 140, 135, 145, 160

The bike probably sat for 10 years before I bought it last year.

 

Yes compression is low , but I thought about something have riden the bike at varying speed ? If bike has sat rings might be sticking , you might add Marvel Mystery oil to a tank of gas run it thru see if it makes any difference.

 

Hi Jetfixer

I'll try the MMO tomorrow... couldn't hurt.

I've ridden this bike about 2000 miles this year. Once I get past 3500 rpm it runs great.

I tried another experi.ent just now - I removed the plug wires one at a time and grounded it so the bike was running on 3 cylinders. I was hoping to find the bad cylinder... However the results were intersting. Each time I started the bike (using only 3 cylinders) it acted the same. I could roll on the throttle slowly but when the rpms hit about 3200, it jumped... but only jumped about 1000 rpm. So the problem is not cylinder specific.

 

do that again but don't shutdown between plugs. just pull the cap up but not off, you'll hear the spark inside the cap.
if you pull a good one off the rpm's drop, if you pull a bad one off the rpm's either don't change or increase.
the extra gap raises the spark voltage and sometimes fires a plug that really shouldn't fire.
swapped carbs or not, i'd spin some mixture screws too

 

So I'm wondering if the problem could be the TCI. I understand that the timing is not adjustable, but could the TCI be partially broken? Is there a way to measure the advance curve? Perhaps the timing is retarded until it hits 3500 rpm.

 

TCI could be faulty but Unlikely scenario

 

I found this info in the forums - Chacal listed it back in 2009.

https://xjbikes.com/forums/threads/timing-curves-for-xj750-its-an-84-model-45t.15193/


TCI:
TID14-21 Hitachi
Initial Timing: 7* BTDC @ 1050 rpms
Total advance: 37.5* BTDC @ 6000 rpms
7* +/- 1* at 1050rpms
9* at 1600 +/- 225 rpms
24.5* at 2600 +/- 300 rpms
30.5 +/-2* at 3500 rpms
37.5 +/-2* at 6000 rpms

So now that I have this information - how do I measure the timing? There aren't timing marks on crank are there?

 

https://drive.google.com/file/d/0B4...ZDFlLTg0YzgtNDU1ZjFlMTRhNjFi/view?usp=sharing
i use this to set the valves, it only has 0, 90, 180, 270 but it might show you enough, like a big jump at 3500.
never tried to use it with the engine running though. it should print the right size, transfer the marks to your plate and make some of your own.
why not try the tci off that other bike and see what happens

 

Polock,

Good suggestion to just swap out the TCI from the other bike... which I did... which solved the problem. Now the red bike runs strong and does not jump at 3500 rpm. I'm gonna have to get another TCI for the blue bike. Anyone got an extra they wanna sell?

If anybody can offer a theory as to why my TCI behaved this way, I'd like to hear so I can learn. I've always read that the TCI either works or it doesn't.

Once again thanks for all the ideas and help in solving this issue.

Rustee

 

I have one for a 750 maxim---

 

Dave,

Thanks for the offer - I actually found one on ebay last night just after I posted this - but if that one breaks, I'll keep you in mind.

 

Was just thinking of something , since you swapped carbs and it did same thing the common demoniter ...have you checked the throttle cable? Try to start bike reach in and move linkage to increase rpm see if it jumps like it is now , if it does not then you will know there is a problem with the cable...might be that simple

 

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