Topic: valve guides

[thenorm78_750f]

I have a 1978 CB750F and have had problems starting it when it was cold and smoked some (sometimes it would take me a half an hour to get it to start when cold. I was lucky the battery lasted that long), so I tore it apart and replaced the rings, and ordered a gasket set. While it was tore apart I relized that all of the valve guides on the exhaust side were extreamly loose, so I took the head to a machine shop to get new ones pressed in. Apparently they are not easy to find; the machine shop had a set made up for it. Finnally, I got the head back and started to put the thing back together.  When I was putting the head back together, the threads for the studs that hold down the cam, stripped out. I bought some heli coils put them in and am now waiting for a new head gasket. Hope to have it back together next week when I get my 2nd new head gasket.

Now that I have explained myself - the questions: Is it a common problem for the valve guides to where out? Has anyone been through this before, and with what results?

P.S. It has about 14,000 miles in good to very good condition.

[Sam Green Racing]

Welcome to the forum Thenorm.

Have a word with Mike Rieck, do a search for Mrieck and send him a PM.
He is red hot on heads and should be able to answer your question.

Sam.

[bryanj]

750 F2 (black engine) VERY common for exhaust guides to wear. Originally bad material plus differences in valve angle overloaded the guides. If it smoked within first year Honda replaced therm under warrantee with no question but they did not recall all those that didnt do enough mileage to wear out. At the time i convinced them to repair a two year old low mileage one but that took some arguing. All the other models were OK I thought

[Lumbee]

...thats scary...I'm got the same bike with 30K...no problem yet, but if it starts smoke'n, I'll know what to check first...

[superchode]

the exhaust valve guides on my 78 750F need replacing as well. the bike was burning oil and the cylinders, pistons and rings checked out OK - the exhaust guides had way more play than they should and that's likely where the oil was getting into the combustion chamber.

i need inlet valves first (hell of a time finding them) - the next job is getting new exhaust valve guides pressed in (luckily those are stock honda parts). after that i can get this beast back together!

my mechanic used to have a couple sohc4 750s and boxes of parts for them. he's very familiar with the engines and the valve guides were one of the trouble spots with them, according to him.

[Lumbee]

...just a suggestion for all with 78F's.  I see the 78F heads on Ebay all the time...much more than previous years.  Obviously this is fraut with peril due to the very issues we are discussing.  However I also know that the F's had issues with weak valve retainers/keepers.  So some of these heads could have conceivably been parted out due to reasons other than the valve guides.

[superchode]

i haven't been watching the bay as closely as i perhaps should be. thanks for the reminder - i'll be more vigilant. often ebay is no more of a gamble than the local wrecker - and the prices are better on the bay (usually).

[thenorm78_750f]

So there you go. It seems to me that 77 and 78 make for a difficult time to repair, being that they are rare compared to other years. Hope that all wiil be well, once I get it back together

My parts just came in today, so I should be breaking in an engine sometime on sunday. Any sugestions or tips?

[Lumbee]

...the springs are a SOB compared to the K...stiffer, so just take u'r time...

[Zeke]

The #1 guide in my 80 650 (yeah, it's a sohc based on the 550) went out in November.  Ha, by the time I rode the 30 home the entire left side of the bike was completely covered with oil film.

Anyway, the guides are getting hard to find.  But, not to worry -- there are alot of competent machinists around that can make them.  They were able to find me both a discontinued valve and guide on the Honda parts locator.

Valve seats had to be recut -- not just lapped to repair that damage.  All others were fine, the machine shop was unsure why the failure.

Just lucky, I guess.

out

[MRieck]

So there you go. It seems to me that 77 and 78 make for a difficult time to repair, being that they are rare compared to other years. Hope that all wiil be well, once I get it back together

My parts just came in today, so I should be breaking in an engine sometime on sunday. Any sugestions or tips?

The 77/78F's are no more difficult than any other CB head to repair or modify. The biggest problem with those heads are the retainers which crack and split if the engine is spun to hard to frequently.

[bhkrause]

The 77/78F's are no more difficult than any other CB head to repair or modify. The biggest problem with those heads are the retainers which crack and split if the engine is spun to hard to frequently.

Ugh, definitely a problem.  First hand experience with this.  If your valves are floating on an F, fix the head, don't wait.