Carb help

[Windy]

Hi, I’ve got a 1982 750F DOHC & have just sorted the valve clearances. The bike runs better now but the revs are all over the place & it doesn’t go back to tick over immediately and it’s running rich. I’m presuming the carbs need overhauling. Now, do I have to break them down to individual carbs or can an overhaul be done with them in a bank of 4. I’m thinking of getting kits from David Silver spares at £90 for 4 . Or I can send them away to be done at a cost of £300 !!!!!! (Don’t think so).
What do you reckon? Cheers Windy

 

[SeattleRod]

Since the carbs are out, I would do a complete overhaul and separate them, clean them really well, and get new rubber boots, intake and exhaust.
I tried to cut corners with mine, I ended up taking the carbs off twice before I took the time to do a proper job. It paid off, I haven’t had the need to mess with them and it idles at 1000-1100rpm.

 

[amc49]

You cannot do a competent job leaving them together, the aircuts on the sides often are the cause of rpm variance issues. The chokes have to remove doing that. The screws are staked and will tear the shafts up removing them. Use a dremel to grind the back end of screw off level with shaft and then the screws come out easier and are often reuseable with loctite to hold them in place. Don't remove all screws, the carbs break in the middle and into pairs and then you remove screws off of one plate only of a pair and leave the other one still in carb body. I remove the screws off the middle pair (2&3).

There are people who say you can remove all the aircuts without breaking the rack down, I personally am not going there.

Be aware that the intake manifolds cook from the heat and if not cracked already to make your idle and variance issues already will likely do so once you yank the rack, it cracks them worse.

 

[Windy]

Thanks guys, that’s what I’ll be doing then. I took AMC’s advice re valve clearances so I’ve downloaded & printed the McGregor guide which is 42 pages of comprehensive info. Guess it’s just a case of being careful & methodical.
What size jets do I need for a stock rebuild? ( using the air box, not pods). Just so I get the right kit for my bike. Cheers Windy

 

[SeattleRod]

I believe stock 750’s run 68/102 jets.

 

[dirtdigger]

I would run the jets that are in the carbs now unless they have been changed. I have had issues with non factory cheap rebuild kit jets. Odd number designations on them and you dont know what size they actually are. Unless they have been physically drilled out the jets dont need to be replaced just because they come in a kit. If you need to pull a bowl and see what sizes are in there.

 

[Windy]

Thanks dirtdigger, so should I buy a kit without jets then? Just o rings, springs, gaskets etc. Would save me some money.
What do you think?
Cheers Windy

 

[dirtdigger]

Unless they are physically damaged there is no reason to replace them if you are just going stock. I never use jets from rebuild kits.

 

[Windy]

Thanks for that matey, I’ll take the bowls off & have a look. Cheers

 

[amc49]

That all sounds good to me if the jets are in good shape, they are better than anything you can buy except for official real Keihin parts. The carbs gotta be pretty bad to scrap the jets although ethanol can do it. The sizes listed in post #5 will work, the DOHC changes maybe 2 numbers max over all the 750s and the carbs being CV type will auto meter to a certain small extent anyway. They self adjust and why you can even use the 1100 ones on a 750 pretty much.

FYI, with euro carbs you may well not have an accelerator pump on that one.............and the pilots should be the screw in type rather than the earlier pressed in ones. Important because at times the top cross air bleed passages on top of the pilot idle feed restriction can clog and you can't get to them to open them up on the pressed ones. The smallest wire a guitar uses, the "E" string, is perfect for chasing the idle feed restriction out to open it.

 

[Windy]

Thanks amc, you know your stuff don’t you?
I was thinking of buying an ultrasonic cleaning machine to do the carb bits in, I’ve also got a Morris Mini van that I’m restoring so I reckon I’ll get some use out of it. Has anyone experience of one of these ? Do I need one with a heater? Thoughts please? Cheers Windy

 

[Chris in Dorset England]

Thanks amc, you know your stuff don’t you?
I was thinking of buying an ultrasonic cleaning machine to do the carb bits in, I’ve also got a Morris Mini van that I’m restoring so I reckon I’ll get some use out of it. Has anyone experience of one of these ? Do I need one with a heater? Thoughts please? Cheers Windy

Hi way back in the early 70s I had a Minivan, if you like getting cold don't have a heater! I don't think they came without one. Mine had the 998cc engine but still had the magic wand gear lever. Fun to drive, terrific road holding! went to Ireland on a fishing trip with it.

 

[amc49]

I've never had need of one but I know people rave about them. I have access to high pressure air at all times to forcibly blow things out.

One thing you wanna watch is the mixing bowl that you can see the seal plug for under the front of the carb, the silver plug right behind the mixture screw location. That bowl takes idle and off-idle fuel to apply it through the miniscule progression 'transfer' holes you find right behind the butterfly edge with it closed. They supply fuel right off idle like miniature accelerator pumps to be the barely off-idle fueling method until the primary system comes into play and the bowl can plug really hard at the same time the pilots do when fuel dries up with bike sitting. Ethanol if used there adds hugely to that plugging problem as it makes fuel dry up like 2X as fast as it did before and the chemicals in it then add to make any varnish remains harder as well. The result being that people sometimes have to clean and reclean the carbs even though thinking they were well done the first time.

The aircuts are for decel and to stop popping in the pipes while slowing down as a result of super lean emissions idle setting, they can be defeated to remove a problem area. Pg. 39 in the carb manual.

 

[Windy]

amc, yeah I read that & was thinking of bypassing them as suggested in the manual. Seems like an added complication to me! I have a compressor in the garage, so can blow jets out etc. It’s just that an ultrasonic cleaner might get to places that other methods can’t. If I bypass the air cutoffs & don’t buy the set of rebuild kits for them, I can save £60 & use this to go towards a cleaner. Anyone used one to clean these carbs? Cheers Windy

 

[Windy]

Hi Chris, my van is a 1972 with a 1275cc engine, Weber carb, cooper s discs, hi lo suspension, straight cut gearbox, big exhaust etc. It’s a full nut & bolt resto!! So I’m trying to get the bike sorted & on the road so I can ride it this summer, then I can concentrate on the van. Lots to do!! Cheers Windy

 

[Chris in Dorset England]

Not quite standard then!

 

[Windy]

Hi people, I’ve broken the bank of carbs down to individual carbs & I’m stripping them now. The jets are numbered 107 & 68 in all 4 carbs. My questions are: is the 107 jet too big? Standard should be 100. Also, in 2 of the carbs the jets are in different positions to the other 2, So which is right? Which orifice should the 100 & 68 be in? No wonder it wasn’t running right. Previous owners eh? Help please!!! Cheers Windy

 

[Wez_]

Read this write up word for word. Don't skip a single page until near the end where it goes over extra processes. I fully disassembled my carbs, soaked them individually in solvent for 24 hours, baking soda blasted them, soaked them in solvent again and lastly washed them in the dish washer on hot and I STILL failed to clean some locations noted in this write up. If you follow this write up word for word until final assembly, consider your carbs mint.

http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revG.pdf

It may seem overwhelming, but download the pdf to your cell phone, and while on the shitter or any other time, read 5 pages at a time and soak it in. I read all 41 pages at least 4 times before I was done with the rebuild. The write up contains all the info you need for jets, air cutoffs etc etc. Its like a mini carb manual.