Topic: KEIHIN carbs

[Serge]

Dear All:

I want to take apart KEIHIN carbs I have on my 1978 750f and I have couple question for you:

The symptoms:

When cold, it dies quite often when go from idle. To start it I had to crank it with a throttle wide open - so I guess it flood the plugs. The idle speed increase drastically as bike warms up, and it revs at 2.5-3 k rpm even with a choke closed.

I would like to buy the rebuild kit and some tools before I take out the carbs and I have found the following ID on the side of the carbs: KEIHIN PD42AAPI1 28, but I can not find out what kit I need to buy.

Any advice/links on how to identify these carbs and what to look for?

In Honda dealership in Sacramento they told me that they will not even take a wheel to replace a tire off the bike so old so I guess I should get equipped to do my own tuning . I found in manual that I will need the manometers or "carb stix" to synchronize the carbs. I also read that having a plug with window ("colortune") is a time-saver when setting up the right mixture without exhaus gas analyzer. Any advice on how useful these things are and where is the best place to buy them?

Thanks a lot !!
Serge

[Matt at PSB]

Got compression? If not then have the valves been adjust recently? If not then adjust the valves clearances & take it out for a short ride. How are the plugs now?

Matt at PSB

[Serge]

thanks for the quick reply!  I have the bike for two weeks now and did not adjust the valves yet, since compresson (dry) is 135, 150, 135, 135 psi which seems within a range... Is it too low? I thought since the problem only at low rpm it must be the carbs...  I checked cylinder #1 when adjusted a timing chaing tensioner, and it seems to be fine. the bike is quite responsive to throttle at higher rpm though there is some vibration above 4500-5000 rpm. May be it is normal though, it is my first bike so i do not have much feel for this yet... Compare with one cylinder dirt bike I rode on MSF course it is nothing . ...

[stanghater]

www.bikebandit.com is pretty good for parts

[Kile]

Howdy Serge,

I've got a '78 CB750K I have been working on for the last few years.  I haven't done a whole lot with bikes before either, so I'm in the same boat.  I'm about 2 1/2 hours north of you in Redding - and the Honda shop here in town said the same thing when I asked them to check the wheels on the bike for me.  They wouldn't touch it.

The bike sat for almost 10 years in my dads garage before I talked him into letting me revive it.  First thing it needed was a carb overhaul.  I got the carb kits from the honda shop - that was a mistake.  Cost me a fortune!  You can find them online for about $20 a pop, minus the floats, which run about $25-30 at the honda shop.  (I haven't been able to find the floats online yet).  The '77 and '78 carbs are the same, and I think the F & K models had the same carbs as well.  (A buddy I work with has a '77 CB750F and the Keihn carbs are identical to mine).

I think I saw the kits online at www.partsnmore.com, but it's kind of hit and miss there.  I don't see any in stock for '77-78 right now.  You might email and ask them if they can/are going to get more. (They currently only have kits for the '71-75 Ks, and '76 F1, though they do have float needls you'll need and the accelerator pump diaphram for your #2 carb).

I didn't have the tools to synch the carbs after I got done with the rebuild, but there are at least two places in Redding that will work on the older bikes.  When I got everything back together, the bike was running well enough for me to ride it to the Yamaha shop and they synched them for me.

- Kile

[Buffo]

I am rebuilding the carbs on my 78 cb750k8 and Keyster has a new kit out for my bike. totally complete. you will need 4 of them the number is KH-1157N.
Easy so far. I have the 1 and 4 carbs done.
Im useing berrymans chem-dip and it works real good. Just be VERY careful with it.

Don

[Matt at PSB]

thanks for the quick reply!  I have the bike for two weeks now and did not adjust the valves yet, since compresson (dry) is 135, 150, 135, 135 psi which seems within a range....

No, those numbers suggest your compression is fine & I am not trying to say it is not your carbs that are the problem. I am trying to get some easily overlooked simple things out of the way first before you tear into the carbs.

Next is the spark plug resistor caps. Pull them off the plugs & unscrew then from the wires. Put the probes on the ends of an ohms meter leads into & on the metallic terminals of the removed spark plug resistor caps. 5,000 ohms plus or minus 20%. Out side this range replace the spark plug resistor caps. They are $3.25 each & I have them in stock.

This is all simple low cost stuff you should always do first before you rip into the expensive carbs. Working on those particular carbs is not fun so lets not go there if we do not have to.

Matt at PSB  

[Serge]

I am rebuilding the carbs on my 78 cb750k8 and Keyster has a new kit out for my bike. totally complete. you will need 4 of them the number is KH-1157N.

Thanks! I found Keyster's kits on siriusconinc.com but they list KH-0983N for cb750f 1978 as "sold out" ...

[Serge]

I didn't have the tools to synch the carbs after I got done with the rebuild, but there are at least two places in Redding that will work on the older bikes.  When I got everything back together, the bike was running well enough for me to ride it to the Yamaha shop and they synched them for me.
- Kile

Kile, thank you for the link. BTW,  how much did they charge you for carbs synchronizing at the shop? - Serge

[Kile]

Serge,

The invoice was $66 - 1 hour of work.  But if I remember right, that included them pulling the carbs to look at a problem I was having with the throttle return spring at the time, as well as lubing the throttle cable.  This was in Nov. of 2002.  At the time I think they quoted me 1/2 to do the synching ($33), not sure if it has changed since then.  I've never synched carbs before, but I don't think it's hard.  I just didn't have the equipment to do it.

Also, I found the receipt fot he carb kits I bought from the Honda shop:

6/1/2002
HO HHP Packing Set (carb kit) P/N 16010-405-004  $16.50 each
HO HHP Valve Set Float (float) P/N 16011-382-004 $26.49 each

Those were for a 1978 CB750K

I was surprised that those kits were actually cheaper at the Honda shop than I've seen them for online (~$20)

- Kile

[eldar]

If you have this problem ONLY when cold, check your choke setting. Make sure when you use the choke, that is closes and opens ALL the way. You will almost always need choke to start since these bikes came out after emissions standards started and they are made to run lean.

Working on these carbs is easier than the previous years. We cannot easily change jets and our main needles are not adjustable. We also have fiber floats and as such generally dont need to worry about the float going bad.

Here is a basic checklist I have come up with after monkeying around with my bike for 2 years.

Step one is basic. Check timing first, always. You cant do ANYthing with poor timing. This includes checking your points and condensers and you may as well replace them if you dont know when it was last done. Could have been 10 years ago.

Adjust timing chain. Check valve clearance.  Check plug caps and install new plugs if you have not done that. Use d8ae to start with. MAKE SURE THERE IS NO "R" IN THE NUMBER.

These are all simple and usually take too much time unless you are inexperienced.

Moving on to more difficult items.  If this bike sat for even just a year with gas in it or longer with nothing in it, change the oil if not already done.

You WILL need to pull the carbs and clean them.  Berrymans is good but you must be carefull to remove ALL rubber parts. A good alternative is a carb dip from Yamaha. It is safe on rubber.
You will proably want to get some parts for the carbs. If your parts are in good shape as in no deep scratches and very little wear, I would not get a carb kit. Get float bowl gaskets and float needles.

Clean your jets well but make sure you do not damage them. During reassmbly, use a 1/8 inch round, smooth piece of metal and make sure the openings of all 4 slides are the same. This will roughly sync your carbs.

 Once you put everything together and back on the bike, make sure all your cables are hooked up correctly. Make sure your rubber boots on BOTH sides of the carbs are supple and undamaged.

Set you idle screws to about 1 3/4 turns out from a LIGHTLY seated position.

This will get you on a good path.

As for your wheel, try another shop. Dealers are in for volume. They dont want to mess around with our old bikes cause no one there knows how to work on them any more. Half of us in this forum are probably better at working on old bikes than most people in dealers.

A dedicated motorcycle shop will most likely help you out.

[Serge]

thanks a lot for all the advices !  - serge