Results 1 to 10 of 10

Thread: Engine Surging

  1. #1
    CB750 Member ulyberm72's Avatar
    Join Date
    May 2020
    Location
    Columbus, Ohio
    Posts
    5

    Engine Surging

    I'm interested in getting a honda cb750f supersport, and I found one listing on craigslist.

    Under the description, he mentioned that the bike has problems at higher RPMs so I messaged the seller and asked him about the "higher RPM issues" to which he replied with a video showing some engine surging at around 4500 RPMs. He has the throttle wide open and the RPMs just bob up and down from 4000 to 5000.

    My question is whether or not I should follow up on this specific motorcycle. Is this issue easily fixable? How much should I pay for a motorcycle with this issue, if I plan to fix it?

  2. #2
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    2,909
    You don't mention the year model and some are wildly different. If '79-'82 often the not pulling out to full rpm is evidence of valve problems because nobody ever sets them. They can even be burned to have to tear engine apart to fix. The valves set by changing shim thicknesses and nobody ever wanted to pay to do that and finding correct shims is a problem too. Why the bike dealers even avoided it when possible to shove the bikes out with 'the valves are OK' when they weren't. Surging while not pulling out to max rpm is a mark of that.

    Obviously there is no way to wrap a cost number around that. That early DOHC is very expensive to fix usually too.

  3. #3
    CB750 Member ulyberm72's Avatar
    Join Date
    May 2020
    Location
    Columbus, Ohio
    Posts
    5
    Itís a 1980 supersport. is there any way of crossing off the possibility that the issue may be a carb or timing problem when I go see the motorcycle?

  4. #4
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    2,909
    You can pretty much forget timing issues of any type if the engine is hitting on all 4 cylinders. The ignition timing is electronic and does not vary and the chains don't jump either.

    The carbs cannot be weeded out at all because they are the CV type and engine condition from the valves not set right then makes them not open and why the engine does not rev up. Basically the engine makes the slides not open. The carbs if anything wrong on their own make the engine not idle right.

    Can't overstate the valve issue, how most of the engines die there. The common denominator there is the engine begins to run slightly off and commonly blamed on the carbs, then the revving gets lower and lower as the compression comes down. The valves not set tend to close up rather than get looser like on most engines and why the compression then begins to leak to kill engine revving up high, the slides dropping from no engine suck then choke the motor off to not rev.

    Ask if the valves were ever set, the answer 9 times out of 10 is no, many don't even know how to on them. They are supposed to check/set every 4000 miles. Everybody thinks they can set them later like on the SOHC but the DOHC will burn valves before then. The valves are very crap quality and recede up higher into the seats like lightning. Why they tighten up.

    If somebody has removed the OEM airbox to use pods instead multiply all that by 1.5, pods make it worse.

    Don't offer a lot, I get them for free running like that. Too much work/money required to get them back right and often an engine teardown involved. The lower it revs the worse it is, they rev all day long at 9500+ rpm when right. Just adding the pods can drop that to 8000, they are a hopdown not up there.

  5. #5
    CB750 Member ulyberm72's Avatar
    Join Date
    May 2020
    Location
    Columbus, Ohio
    Posts
    5
    Thank you so much for your help. I just contacted the seller asking him about setting the valves, he said he did set them when he first got the motorcycle and has only ridden it a few times, he just lost interest in it as a project. Maybe he didn't set them right? I'm honestly kind of steering away from this motorcycle as a project for myself, as I'm still not that knowledgable and don't have the confidence to take apart the top end and set the valve height myself if that ends up being the case.

  6. #6
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    2,909
    If he set them and the engine did not go to revving high again the valves (at least some of them) are likely damaged already. Why he lost interest. But then I knew that as likely already, getting that low in revving is a solid mark of it. Simply setting valves (too late) cannot repair ones already burned, the engine comes out and head comes off to fix that. Typical. And not cheap.

    Look at how you feel about doing the work yourself. Nothing wrong with that, not everybody can be deep engine wizards, but you can now see why so many of that type bike are sitting in garages gathering dust. They were designed to be too maintenance intensive to ever live very long lives, today's owner generally simply cannot (will not) do it.

    You are likely saving yourself some considerable frustration. Let somebody else have that headache, some people like me LOVE self-punishment. I never had issues past burning several valves before 3000 miles on my first new one and no trouble after I understood the problem, but it IS a problem you absolutely must stay on top of and nobody ever gives thought to them being 40 years old now, the thinking is they are like the SOHC and nothing could be further from the truth. They run like lightning but are VERY delicate and damage easy. The SOHC is slightly less powerful but relatively never breaks if you don't abuse them.

  7. #7
    CB750 Member ulyberm72's Avatar
    Join Date
    May 2020
    Location
    Columbus, Ohio
    Posts
    5
    Again I thank you, I honestly would've excited ran out and bought the motorcycle if I hadn't found this forum page. I'll keep looking in hopes of finding a SOHC cb750 now that I know about the valve issues with the DOHC. Just out of curiosity, how much would it be to replace all the valves and shims?

  8. #8
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    2,909
    I can't even put a proper number to it. The exhausts go for commonly $30 each at 8 and the intakes half that but intakes often can be lapped in to save them. Valvework on head if needed $250. The reshim can run another $100 labor bill plus shims at $5 each and cheap at that. Carbs cleaned and bench synced (right) $250. Gasket set $125 bill and the seals are garbage so you buy them over again, another $50. If enough miles on it all tensioners and guides needed, another $250 easy. The engine yanked out and apart and back together running (correctly); I have no idea, probably another $200-$250. It would be a massive bill if you have to pay to get it done. Why those who agree to do it commonly lose money if they've never done one before to sack you like you are a bank. Likely they will mess it up anyway.

    Those prices are VERY rough and likely old to be too low but you get the idea. A bank-buster.
    Last edited by amc49; 05-18-2020 at 09:31 PM.

  9. #9
    CB750 Member ulyberm72's Avatar
    Join Date
    May 2020
    Location
    Columbus, Ohio
    Posts
    5
    that's crazy. I'm so glad I found this community and more importantly that you replied so fast. can't thank you enough.

  10. #10
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    2,909
    Glad to help. The only way to get the bikes running with anything approaching economy is to have the skills to do it pretty much 100% yourself. I love them and can still do it but I don't like to lead others into a money pit.

Similar Threads

  1. Engine chugs at 3000 rpm only after engine is warm
    By eenvader in forum Carburetors / Fuel
    Replies: 7
    Last Post: 07-25-2016, 12:32 AM
  2. 1991 cb750 engine running on just one side of engine ( 2 cylinders )
    By Jayboy74 in forum The DOHC Garage (1984+)
    Replies: 5
    Last Post: 05-23-2013, 10:53 AM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •