Cbr900 Bike Carbs

[Dean Roberts]

I have connected up a cab balancer (courtesy of a good friend) on my CBR900 carbs on a Pinto.

The main Carb according to the manual is cylinder no 2. I adjusted cylinders 3 & 4 and matched them with cylinder no 2 with ease.

 

But cylinder no 1 is very low and not even getting anywhere near what the other 3 are sitting at (even with the adjustment screw fully out).

Could this be a faulty cylinder?

 

The car is driving fine, (no spluttering, stalling).

 

This is basically what it looks like:

http://honda-cb750-s.456789.n3.nabble.com/file/n4003587/AAAAAAAAAAAA.jpg

But the person who reported this is using different carbs and has the issue with cylinder 4.

After reading the forum, there was no solution, so wondered if anybody else has had this issue.

 

I was going to book the car on a rolling road, but don't want to waste my money, if I have a duff cylinder.

I cannot give you much more information, as they have very little history and I haven't had chance to strip them down....Yet!

Being impatient, I just wanted them on and the car running for it's MOT.

 

Any advice appreciated.

[agent_zed]

air leak?

 

have you done a compression test on the engine, should show if a major issue on that cylinder.

 

can you swap the gauges on the tester to make sure its not the testing device?

[Dean Roberts]

air leak?

 

have you done a compression test on the engine, should show if a major issue on that cylinder.

 

can you swap the gauges on the tester to make sure its not the testing device?

 

I was wondering if an air leak might cause this....

But are you talking about the carbs or engine.

 

No compression test or history...so something worth considering (reading about)

.

I have swapped the gauges around and the same carb/cylinder is at fault...so the gauges are fine.

[agent_zed]

i'm certainly no expert and will be entering the fun of bike carbs shortly but my understanding of carbs in general will be that you could have an air leak where the carb mounts either at the carb/manifold or manifold/head. The usual test for this is to spray wd40 around the joints and listen for a change in engine note as the oil will plug the leak for a short time.

 

I have read bike carbs have breather pipes which some people block up so worth a check to see if that carb isn't block and the other are.

 

Compression test on the engine will show if the cylinder has a problem. If its a lot lower than the other cylinders then you have your answer. They aren't that expensive i think i paid about £18 for a compression tester. Easy to do on the pinto. Just hold the throttle open and crank the (warm) engine for like 10secs, disconnect the coil so it doesn't start.

[Guest 2b cruising]

If you are buying a comp tester beware Dino.

I bought a new but cheap off the famous enough bay.

Tests my new Vauxhall engine with it and didn't believe the readings.

Borrowed one from my old workplace and proved the new tester to be 100 psi lower than proven and tested borrowed gauge.

Edited April 22, 2015 by 2b cruising

[agent_zed]

Borrowed one from my old workplace and proved the new tester to be 100 psi lower than proven and tested borrowed gauge.

 

wow that must have been properly wrong. At least that is good it was so wrong you noticed rather than just a bit wrong.

 

I bought this one http://www.toolite.org.uk/petrol-engine-compression-tester-0300psi021bar-p-128228.html which is a draper and seems to be ok. Pinto showing around 160-180psi from memory. The company is local and seems to have a good rep.

[Dean Roberts]

For the essence of impatience - I bet the local garage by my house has one of those and are willing to test it for a couple of beer vouchers.

If not then its on the checklist ?

 

thanks guys.

[Guest peter2b2002]

I think I have a comp tester in my shed if you need one

peter2b

[Dean Roberts]

I think I have a comp tester in my shed if you need one

peter2b

 

I do sir.....Any chance of a beg/steal or borrow?

Edited April 22, 2015 by Dino

[cb750]

Dino, are you adjusting the mixture screws and not the balancing screws which synchronise each carb opening to each other?

[maccavvy]

I thought number 3 was the main ???

[cb750]

You may already know this but my notes say to initially balance carbs (I think mine are the same as yours) screw the mixture screws out by the same number of turns (I think about 2 1/2 turns?) then balance #1 and #2 carbs with their screw. Then balance carbs #3 and #4 with their screw. Finally balance pair 1&2 to pair 3&4 using the centre screw.

 

Rob

[Dean Roberts]

You may already know this but my notes say to initially balance carbs (I think mine are the same as yours) screw the mixture screws out by the same number of turns (I think about 2 1/2 turns?) then balance #1 and #2 carbs with their screw. Then balance carbs #3 and #4 with their screw. Finally balance pair 1&2 to pair 3&4 using the centre screw.

 

Rob

Yep, done that and have no issues balancing the carbs...

Just querying why one of the buggers won't play fair and balance with the other 3...

 

I will take the wd40 out into the garage and see if there are any air leaks.

[Dean Roberts]

I thought number 3 was the main ???

 

Oh u tease.... Now I've got to look at the manual again and hope I'm not a complete spanner

[Dean Roberts]

Notice how the needle sits way below the others, and no matter how much I adjust the screw it only rises up to the top of the red section.

All the other have balanced perfectly:

http://i1275.photobucket.com/albums/y450/Dino2B/cbr900/IMG_1976_zpshvxnadqy.jpg

 

I have also noticed tonight that the vacuum pipe to the inlet is very discoloured for the same carb after a days outing:

http://i1275.photobucket.com/albums/y450/Dino2B/cbr900/IMG_1977_zpswts2t9pi.jpg

 

There was a video on there...but wont play an iphone video....sorry!

 

Any ideas?