Topic: 78 cb750F - at a loss

[aperfcrcle]

hey guys.. well I am completely at a loss with this thing at the moment... I recently purchased it and it was not running.  I have done the following:

- taken the carbs apart, cleaned, bathed them in parts cleaner, cleaned every jet, new gaskets, etc..
- Replaced battery
- gaped the points
- set static timing with a multimeter
- checked the valve clearance
- cleaned the tank out and added a new petcock with fuel filter
- made sure there was spark to each plug

So I put it back together the other day just to see if it would run, and to my amazement it started right up at the push of a button..
It was burning some oil when it first started up (blueish smoke coming from exhaust and around the headers where it leaks alittle).  So I put it away as it was later in the day, went to start it back up and nothing.. it cranks and sputters but does not stay running.  while it sputters some smoke puffs out of the exhaust.  I went back and checked everything, fuel comes out of the petcock no problem, no fuel is leaking anywhere, checked the timing again, checked spark again its all good.  The spark plugs were alittle wet and smelt like fuel..Any ideas why 12 hours would make a difference in running and not running?  I will try to get a new set of plugs but like i said they have a very strong spark..  I am starting to get frustrated.  thanks in advance guys

[bryanj]

78F carbs are PD type and notoriously bad also plus can spark well outside but fail under compression.
From smoke details would just about guarantee the exhaust valve guides are worn out

[Bankerdanny]

How many miles on it?

As bryan mentioned, the 77-78 750F's were prone to excess valve guide wear due to a change in the head design that only applied to that particular model. That said, it isn't unusual for bikes that have sat for a long time to smoke for a while. Sometime the rings are a bit gummed up and don't seal properly until the engine has run for a bit.

Pull the plugs again and take a look at them to see if they are either wet (gas fouled) or sooty. Either could keep them from firing properly.

It can also take a couple of cleanings to really get the gunk out of the carbs.

Don't give up. You'll figure it out.

[754]

Scrape off the 2 outer plugs, see if those will now fire.

[aperfcrcle]

thanks for the responses guys.  The bike has 16k and change miles on it but the last inspection sticker was from 1991 so I am guessing thats the last time it ran.  I am going to run out and see if I can get some spark plugs for it now.  When i took them out they were sooty and smelt like gas as you mentioned.  I am concerned about the leaked around the head, seems like it was just coming from the valve gasket and pooling by the headers, but the more I read the more im thinking I am going to have to pull the engine and replace the pucks. 

[aperfcrcle]

here was a video of it running after a few minutes if it actually posts..

[jaytee-nz]

The only thing that has changed since your initial start up is that you have run fuel through the engine. This could mean that the tank was not totally clean and as the slow jets on these carbs are notorious for blocking up, fine dirt / sediment from the tank has made it's way in to them, causing starting issues.

[evinrude7]

Battery?

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[BenelliSEI]

Try a set of ne NGK plugs....... if that doesn’t work, try dropping a float bowl. If it’s loaded with fresh dirt from your tank, you’ll need to clean out the idle jets and circuits again..... they are so small it takes nothing to plug them.

[aperfcrcle]

alright well, took everything apart again, bowls were clean, jets were clean so its not that.  Looked in the tank and there is some rust forming again so I guess im going to have to seal it so I will do some research on that.  I guess its going to have to be spark at this point so once this whole virus thing blows over I'll get going on that..

Also, any suggestions on how to remove a frozen master cylinder reservoir cap? This thing will not budge.  tried to torch it, liquid wrench and nothing.  thanks

[BenelliSEI]

Did you screw in a new set of plugs?

[Fenris3006]

Are NGK DR8ES-L correct plugs for the F2?

[bryanj]

NO do not use resistor plugs. Also without removing the press in pilot jets you cannot be sure both they and the pathways are clean enough

[Fenris3006]

-but the type/name of correct spark plugs?

[bryanj]

NGK D8EA or NGK D8ES-L

[Fenris3006]

Thanks a lot!

[HondaMan]

Are NGK DR8ES-L correct plugs for the F2?

Those will work fine. In that era Honda sold the DR8ES-L plugs in the CB750K7/8 and F2/3 in Canada and certain Euro countries who required less radio interference in the cars next to you as you rode by. I run those, or DPR8EA9 in my own 750 when riding in town because the resistor makes for a slightly longer spark duration and helps keep the plugs a little cleaner.

The biggest trouble with the F2/3 engine after it sits a long time is: uneven valve sealing. The valves in this engine are easily rusted, which lowers compression on a couple of cylinders (usually the 2 that had open exhaust valves when it sat) for a while until you can fire it up and ride 100-200 miles again. Until that time, you may find you need a box of plugs (or a dozen) to live thru the fouling it will do to them. The running engine will tend to clean the rust off the valves: if the guides have survived then it will run for a while: if not, it will start leaking oil out the exhaust ports and make a mess.

[bryanj]

Hondaman i bow to your knowledge of the US market. We never got the F3 in UK, it was too difficult to sell the F2(still had  stock at end of 79!) As to the oil mess i can attest to that, 150 miles per UK pint, 103 mph flat out and a very peed of Isle of Man constable(inside very big blue cloud)