Air intake rebuild and bad timing

[SlipperyPete]

I was about 50 miles from home a few weeks ago when my ‘78 GS750 stopped contributing to my beautiful Saturday afternoon. I was motoring along a state highway about 55-60mph when I started to feel what was, for a lack of better description, mini-surges. Although I was keeping a steady throttle, the bike would slow only slightly, noticeably though, then resume normal operation. I soon started hearing loud bangs coming from the pipes (a friend later thought this could be after-fire). There was no power and I couldn't get it to go over 50mph. I limped to a friends house where we got it on a trailer and eventually back home. I’m really sad about this because the riding season around the Twin Cities is quickly coming to an end and I don’t have anything to ride.
I’ve only had the bike since May when I got it from a friend. I know he took good care of it for the half of it’s 37K miles he had it, but that it also needs some TLC. I wanted to get through to the end of the riding season and then start on a big maintenance project over the winter. I planned to do the air intake, carbs, petckock, and possibly something else (suggestions?). As it is now, I’d like to do something quick to possibly get it back to running again so I can finish the season. I’m thinking about tackling the air intake because, after reading a lot of the posts and required reading here, it’s a likely cause of the problem and I can do it fairly quickly. I've replace the plugs (they were brown/black on the lower 1/3 of the plug) and that helped a little, but the problem hasn't gone away completely.

Here’s the plan:
Clean the air filter
Replace the boots and clamps on the airbox side of the carbs
Replace the clamps, screws, pipes, and o-rings on the other side
Hope that it works

I’d appreciate any input and suggestions anyone has. Thanks and wish me luck!

 

[Steve]

I get the impression that you have been riding it, possibly a fair amount, but have you, or the previous owner, done ANY of the maintenance that might be several thousand miles overdue? He may have "taken good care of it", but that can be interpreted as keeping it clean and shiny so it looks good when anybody happens to see it.

How many miles have you put on it this summer? Are you aware that valve clearance is supposed to be checked every 3,000 miles? They tend to tighten up with use, and tight valves can cause some of the problems you describe.

I am also going to guess that nothing has been done to the carbs. :-k Yeah, you have been riding it, but who knows if it has really been running right all that time?

What you are currently experiencing could be the result of tight valves, compounded with improper carburation. Either one, by itself, is bad enough, but having both at the same time definitely shows up as a problem.

Please add valve clearance inspection to your "to-do" list, and consider rebuilding the carbs over the winter.

.

 

[SlipperyPete]

Thanks for the reply. I've edited my original post to include the info about no power and not being able to go over 50mph.

I've put less than 400 miles on since May. I haven't had time to ride. I've read about tight valves. I'll put that on the list too. I was hoping to avoid that because I don't think I can do the carbs quickly enough to take advantage of what time I've got left before it's too cold. If I have to hang it up for the season, so be it. I'd be awfully sad, though.

I know I need to do all of the maintenance I've read about. I plan to, but I just want a few more weeks of riding because it's so nice right now.

 

[Steve]

[Clint Eastwood voice] "How lucky do you feel, punk?" [/Clint Eastwood voice]

The part that you REALLY need to consider is that this might be the early stages of your engine doing its own little melt down.

Yeah, you might get another couple of weeks of riding it now, but it will cost you HUNDREDS of dollars to fix the damage that you cause.

Or, you can do the maintenance for considerably less than ONE hundred and be ready for that occasional 'warm' day during the winter.

.

 

[SlipperyPete]

Thanks again for the advice. You're right, of course, I shouldn't push my luck. I don't have any idea how to inspect valves but I'm going to learn.

 

[eil]

If no one has pointed it out to you yet, see the link to the mega-welcome in my sig. We have a vast library of service manuals, tutorials, and everything else operated by our own Mr. BassCliff. Everything you need to know can be found by clicking through and browsing around.

 

[Steve]

Have you been given a set of keys to The Library? :-k

If you can't find it there, it's probably not available. :encouragement:

.

 

[Big T]

Actually, it sounds like a points problem to me
Does your bike still have points?
Do you know what thy are?

 

[Grimly]

Actually, it sounds like a points problem to me
Does your bike still have points?
Do you know what thy are?

Ditto that. After a few years riding bikes with electronic ignition, I was caught out by a set of points on a model I was sure had EI too. Turned out it didn't, and the symptoms were exactly as described once the points wore down enough so they were barely opening.

 

[SlipperyPete]

Thanks again everyone for your replies. I'm at peace now with not being able to ride until I fix this thing.
I don't know what points are and up to about 5 minutes ago even where to look for them. I'll check it out tonight/tomorrow and reply after doing more learning about them.
The valve adjustment looks daunting. I may have to find someone with more knowledge and feed them beer and brats to help me.

 

[Big T]

The points are under the small cover on the right side of the motor

See if they're opening when the crank rotates, try starting it up and see if the points are arcing (best done on dark conditions, arcing os bad)

Read up on how to adjust points and set timing in the factory service manual you just downloaded from the BassCliff site. You may get a few rides in along the St Croix yet this year

The valve adjustment is easy, but tedious the 1st time you do it, because you need to catalog what you currently have. Steve has more than just good advice, he has a spreadsheet to aid you. See his signature. Your friend must have done it, ask him for help.