Today I started by top end rebuild putting on a 351 kit and bolting on a cylinder head I bought off Adrian. When I went to put my carb on I realised the carb holder rubber on the head I bought from Adrian was much bigger than my stock holder.

My first thought was that the port sizes of the cylinder heads were different too, but when I measured them with the veriers I was relieved to see they are the same at about 32mm. The inside diameter of the original carb holder where the carb goes in is just under 40mm if I read the verniers correctly, but the replacement holder has an inside diameter of 46mm. Now, given an FCR 35mm carb fits into my original holder I guess I could be looking at a 42mm carb (if such a size exists).

Any advice on what my options might be? Of course, if I end up going for a larger carb then I am going to have to go through the fun process of figuring out jetting for it with my 351 kit which I'm guessing won't be easy as not many people out there have such a different size carb.

 

is it for the efi bikes ?

 

Yeah, my same question as wildcard applies...what's on there now? The biggest carb normally installed on the KLX is the TM36-68. 36mm is a notable increase, and frankly I think you'd be overcarbed with most anything above that. I've also seen an FCR-37 installed on a KLX but not often. Why do you feel you'd need a 42mm carb? I believe the last carbed Honda CRF450R was only 40mm, and the KLX at any displacement doesn't have near the volumetric efficiency of that particular racing Honda.

Another beauty of the Mikuni 36mm pumper is that the jetting has pretty well been hammered out, so you're not starting in the dark...and...the TM36 is basically a straight bolt-on.

 

You probably have the fuel injection rubber intake carb holder. Just use an original carb style rubber intake and you'll be back to where you want to be in that regard anyway

 

I just noticed that it looks like this bike already has an FCR-35 carb? If so, I think I'd stay with that and be done. 35mm is plenty for the 351.

 

Yeah - I was thinking 42mm just because of the carb mount size. On researching a bit it seems that DRZ400 riders have had success with an FCR 39mm but even then it's probably a bit big, so a 42mm would be insane. It would probably be wiser to aim at an FCR 39mm or an FCR 37mm carb.

The reason I have reservations about keeping my FCR35 is that I did a dodgy-mod to the spigot on the engine side so that it was long enough for the carb to fit. That, and the FCR35 is nowhere near as readily tuneable as the more modern FCR-MX carbs that have more parts available to them.

A couple of knowledgable mates over here have advised me to rather spend some money on having a collar machines up and pressed onto the carb so that it fits the carb holder I have rather than toy around too much with other carbs. It's probably sage advice.

BTW TNC, I need to update my sig - I had the FCR35 installed at one stage but then struggled with the jetting in summer so I took it off an put the CV carb back on for reliability purposes. With putting the 351 kit on I intended to run the bike in with the CV carb and then move to a pumper carb - probably the FCR35 I have at first. But given this spigot diameter problem I thought it may be worth getting an newer FCR-MX style carb with adjustable accelerator pump/leak jet setup.

I have been accused of over-thinking things in the past, and I'm probably making the same mistake again now :-D

 

Arctra, the Mikuni TM36-68 pumper is an excellent carb for the KLX with any big bore kit. It's about a straight bolt-on with the jetting already hammered out. The head and cams on the KLX will probably not gain any flow beyond a smoothbore 36 anyway, though honestly it would take a flowbench to qualify that for sure.

 

Arctra check your PMS I'm interested in that head of yours