Carb port hole puzzle

1980 XJ650 Maxim. It’s running pretty well but I have one odd issue. Cylinder #4 stays cooler than the rest when at idle. I randomly covered the top hole on the airbox side of the carb and the revs shot up. However, if I do that on the other 3, the bike basically dies. Can someone help me understand a) what the hole does exactly and b) what it means if one reacts differently than the other 3?

thanks

 

Hey @jekyllman,

I'm pretty new to the forum, so I'll preface this by saying you might want to take my advice with a grain of salt, but I'll spitball some ideas to start off, hopefully some more people will jump into the thread as well.

From what I know, that hole you're plugging brings air to the lower portion of the diaphragm, so when you plug it it's forcing the diaphragm to drop, and running fuel via the pilot jet instead of the main jet (since the piston attached to the diaphragm is dropping and shutting off the main fuel circuit).

My first question is, have you cleaned and rebuilt your carbs, and set your valve clearances? if you read through posts on here, that gets hammered into people time and time again, cause your bike simply won't run right if you don't get those 2 things in spec.

Once you've done those, if I were to guess I'd say cylinder 4 racing when the pilot is on (and it's cool running nature) means it's running rich. If you haven't wet set your float levels, that's super important and should be the next thing you do. If you have, your next step should be setting your idle mixture with the brass screws at the front (I'm not 100% on the location on the Hitachis, someone want to calrify?) of the carbs.

I mention this because if you haven't taken your carbs off the bike, disassembled them, and rebuilt them, you have ZERO idea what a previous owner did to them. I'll attach a pic of what greeted me when I first opened up the carbs on my bike (which ran, but not well).

If you've already done all of that, or are confused as to anything I've mentioned, let us know so we can move to the next step in getting your bike running right.

Good luck!

 

Had a similar issue on my 1980 XJ650 (UK) model. It was only idling on three cylinders, #3 cylinder wasn't firing, could grab hold of the header pipe and it was cold. Tried adjusting the "Do not attempt adjustment" pilot mixture screw and "hey presto", it started to run on all four cylinders. Can't remember whether the screw was turned in or out. After that, had the idle mixtures set up on each carb. using exhaust gas analysis from the tapings on the header pipes before they go into to 4-2 collector box.

A plug check revealed this, as posted above, #3 cylinder was running rich.

 

This is interesting, ask yourself what's under the diaphragm? Not very much, except an air jet - or is there 2 on the Hitachis? One for idle and the other for main? It could be that your mixture is miles out rich, and making it richer does nothing. Or, you have a hole in your diaphragm?
The piston will be sitting on the bottom at idle, so shouldn't move when you cover it.

 

So I rebuilt the carbs and have cleaned them out a few times (another issue with gas leaking into the oil). New jets, adjusted the floats, etc. However, I have not checked the valve clearances. Let's just say taking the cover off and messing with things in the engine makes me nervous. I will say that I had aluminum foil covering the vacuum port on the carb boot and it popped off.

I was going to look into adjusting the timing but can you adjust the timing on these bikes? The manual didn't provide much there.

I've also got those "do not adjust" screws for the pilot mixture at the same place in terms of turns from fully seated. So each of the carbs should be quite similar from a richness/leanness standpoint.

Not sure if that helps with narrowing our focus here but I'm not sure I know what to think of this at all.

 

timing is not adjustable.
covering that hole allows more raw fuel to flow into carb throat.
the mixture screws ( do not adjust screws)are to be set 2-1/2 turns up from a soft bottom. open the mixture screw up another 1/4 turn on that cylinder
that hole is where the airjets get the air from. check to see if you have the air jets reversed or if you have holes in the diaphram


if you can clean carbs you can check valve shim clearances and adjust them.

Bigfitz's AIRHEAD VALVE ADJUSTMENT with Pics - parts I & II

 

^+1

I just finished doing my valves a month ago. In terms of difficulty, it was the same level of "holy s*@!, this is scary" as taking the carbs off the bike was. Only difference is it's on a different part of the engine, and it requires a little more time, but it's no more difficult than that.

Just make sure not to drop anything down the case, and you'll be fine. Bigfitz's valve guide will be your bible, it was mine for that job.

Also, don't discount doing your valves. When I opened my cover up, it turns out I had negative clearance on one of my intake valves (so it was always a bit open), and every. single. valve. was out of spec. Turns out that's why my bike didn't run right. Huh, almost like the wizards on this site know what they're doing When someone who owns 4 of these bikes gives advice, treat it like gold, and it'll take you far.

Let us know what your valve clearances are at, if you get confused as to how to do a check I'm happy to walk you through it (as are others on this site).

 

Wait a minute here now, you cleaned the carbs a few times? And yet you didn't blow out the mixture screws ports?
Methinks you need to do this again, remove the screws, check all fuel ports again. I'm not a big fan of new jets - unless you buy new genuine ones you can never be sure of the quality. What you're doing is akin to washing the bike when you have no petrol tank.....
Ps, check valve clearances

 

"I will say that I had aluminum foil covering the vacuum port on the carb boot and it popped off." Hopefully you have replaced any aluminum foil with the proper rubber seals. They don't run well with those valve ports leaking air.

 

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