XJ900 Rebuild - First startup

Hi Everyone,

My 900 project is nearing completion and yesterday I took the bold step of trying to start it up. For the record the following work has been done on the engine:

* Carbs dismantled and all parts and passages cleaned. New rubber diaphragms. All 4 slides passed the "clunk" test. New fuel T piece between #2 & #3 carbs. All floats smoothly operational. Bench synced to within a millimetre of its life. Installed and sitting dry on the engine for around 6 months
* Shim clearances checked and adjusted to within spec
* New Cam chain
* Timing of cams etc. spot on as per Haynes manual
* New coils installed a couple of years ago
* New spark plugs and a good healthy looking spark on all 4 plugs.

No tank on yet. I've rigged up a metre (1 yard approx.) fuel line and have filled it with petrol until the carbs were full and fuel was building up in the clear fuel line.

OK. Fuel enrichment on and...Press START and...she fires. A bit lumpy at first but seems to settle but with a bit of a miss which never improves. Starts to bog down.

An inspection of the plugs show number 1, 2 & 3 a nice light tan colour. Number 4 is black and sooty. I swapped plugs 1 & 4 just to make sure it wasn't a faulty plug and the same thing happened on number 4.

Short of removing the carbs to make sure that number 4 carb is set up as the other 3, any hints as to where to begin to look for the solution? Please note...this was a running bike before the strip down so TCI etc. is known to be good.

Thanks guys...I've received some great help on here during the rebuild!

 

did you check the fuel level in the carb bowls Wet set?

Setting the fuel levels

pilot jet mixture screw adjustment set to 2-1/2 turns from soft bottom?
test for spark with spare plug groundes to motor.
make sure enrichment plunger is fully seated when choke off
running sync the carbs or just adjust carb 4 sync screw to see if it improves
but sync the carbs
3 more links
How To: Make a 2 bottle synch tool

http://www.xjbikes.com/forums/threads/5-carb-sync-tool.4191/


http://www.xjbikes.com/forums/threads/old-school-method-of-one-vacuum-gauge-carb-sync.2132/

 

Hi,

Thanks for the feedback.

Pilots screws out 2.5 turns as a starting point. Yes.
All 4 plugs tested visually grounded to the motor.
Enrichment plunger. Looks OK visually. I replaced the small rubber boots on the outside of the plungers. Some were a bit sticky but I think I sorted that. But I will revisit now you mentioned it.
I did not wet set float heights. If all else fails the carbs will have to come off and be inspected )):-(( grrr

 

This is probably the problem.

Gary H.

 

you can check the fuel level with carbs on bike put it on the centerstand

 

OK...limited time today but a cursory check seems to lay the problem at the feet of the #4 enrichment plunger. These were all cleaned when the carbs were apart and new rubber caps placed on the outside with the hex retaining nut.

I recall over 6 months ago when I reassembled the carbs that manually operating the rod that activates all four plungers that #4 took it's time returning to it's closed resting position. This could be friction from the new rubber seal cap and a small spring made softer by 30 years of operation.

It's almost always something simple...just need to isolate which one or combination of factors is the cause of the problem. Also...when working on a project over a longer time frame means that assembled carbs are not installed straight away and even if they are it's been a good 6 months after that before they were filled with fuel and started up. You kind of lose the continuity of thought and forget that a small problem on the bench may be the cause of another problem in practice.

Wil report back when fully tested and confirmed.

Thanks for the contributions!