Vacuum Guages

[Oliver Boy]

Hi guys.
I'm starting a carb improvement project; my '81 has never started very well and I'm tired of hanging onto the throttle for 10 minutes every time I start it up.
The bike is now going have to be a daily commmuter -rain and shine- it's got to be more user friendly.
After checking valve clearances, timing and cleaning the carbs, I intend to drill and adjust the idle jet (I'm told it runs to lean on modern fuel) and then balance the carbs.
Am I missing anything ?
Can anyone recommend a good set of vacuum guages?

 

[amc49]

You will almost certainly screw the jets up you touch with a drill. While I have done it on cars, the sizes bikes use are so much smaller you generally screw the part up. Idle jets even more so because the standard idle jet on a 750 Honda is commonly around a number 35-38 with the hole size being around .013" in diameter. (1), have fun finding bits that small, they DO make them but not on every corner. (2), the difference from that bit to the next size is about a full one thousandth but actual drilling can without so much as batting an eye result in another thousandth or more in error simply from the finish the bit leaves behind as no two holes drill exactly the same. Getting all 4 the same flow? Hey if you can do it then you need to enter the lottery, you are a sure winner. All jets that small are not only drilled, the surface finish and radius into and out of the hole makes them all flow different from each other. A drill bit invariably introduces varying amounts of swirl into the hole, the OEM jets are commonly finish burnished or polished out to be dead round to make them much more even with each other.

If you want to richen then go the next step up, check your idle pilots for a stamped number of 35 and if so then order 4 #38s which is next size up. Those carbs if they are the right ones should have screw in pilots instead of the earlier pressed in ones.

Remove either the tabs or the cap itself from the idle mixture screws, the cap is loctited on and heating it will let it come off, careful, the screws break easy as spit at the narrow on them. Or you can grind the tabs off the fuel bowls to leave a handy tab on the setscrew, makes it easier to turn them under carb. What I did. Then set them at around 2 1/2 turns, a bit richer.

If you don't like holding the throttle then get some sort of 'cruise control' device like a Vista-cruise throttle locker, I highly recommend one for warmups. In fact, I won't have a Japanese 4 cylinder bike that does not have one on it.

Hope you set the valves toward .005" as a standard, if you used the book .003" number the bike idles worse.

 

[Oliver Boy]

Thanks for the quick reply, my post was a bit ambiguous
What I meant to say was that I intend to access and adjust the idle mixture screws, I read that I would do that by drilling the covering off - just as you suggest-
I'm certainly not about to attack my delicate little places with a drill ��
Thanks for the heads up about the valve clearances.
One of the things I am hoping to cure, is a variable idle that has become a problem recently. I made up a sort of throttle lock for starting the bike from cold (I used elasticated velcro, it actually worked pretty well) but after only a few seconds the revs would either increase way too high, or decrease and die -so I figured it was time to sort the whole thing out and educate myself in the process
Any thoughts on which vacuum guages I should order? Or what I should be looking to pay for a decent set ....

 

[amc49]

I don't use vacuum gauges, rather a set of parallel tubes ala Carbstix. Vacuum gauges are never calibrated to be exactly even at the price break you will be looking for, they are not accurate enough, at least for my liking. The Carbstix type of thing is by virtue of how it works self calibrated instantly, all the cylinders are read to be accurate. Of course it involves using mercury in them, big woo, a poison there.

There is no cover to drill through on an '81, they use external idle limiter caps, a tab strikes another tab to stop you moving the setting further. One of the tabs has to disappear.

Compression check in order, those are well known for variable idle due to loss of compression, why I mentioned the valve setting. The idle moving around a lot is one sign of bad compression. Not talking hot vs. cold, talking hot and warmed up.

 

[Oliver Boy]

Thanks amc49
This job is getting bigger rather than smaller but I am learning a ton and I appreciate your time.
Carbstix eh? Right. I'm on it...
I have a few hours off today; valves first I am thinking.
I used the bike quite a lot this summer, apart from the reluctant starting it runs like a train- really strong with no idle variation once warmed up.
Starting deteriorated quickly and I have convinced myself that most of my problems will be solved with a really good carb overhaul; but I am not inclined to make do with a running fix- I want to set it all back to perfect.
Another issue I must address is a leaky exhaust valve seal. (One of the exhausts smokes a bit during warm up, annoying but not excessive and it clears up once the engine is warm). (I have a spare engine I want to overhaul, then I intend to swap them over and take the top of my current one for the valve work)
its probably a silly question but could that get so bad as to effect compression on that cylinder?
I saw a hack on youtube whereby valve seals can be changed without removing the head by stuffing the cylinder full of string at TDC. What are your thoughts on this practise?
Thanks again for your helpful replies. If you lived next door I would buy you a pint :
Oliver boy.

 

[amc49]

I yank head to change seals. Common for them to get hard and leak, but not to affect compression, the valves do that.

Thinking Carbstix as a brand do not exist any longer, the mercury requirement killed them. You can't send mercury in the mails any more.

 

[amc49]

https://www.denniskirk.com/motion-pro/syncpro-carburetor-tuner-08-0411.p282760.prd/282760.sku

Same idea but does not use mercury. Technically called a manometer.

Root around in search using 'carb sync' as a term, the 4 vacuum gauge thing shows up too.

 

[Dave]

First, You should "Un-Convince" yourself it is the carbs without solid evidence. When we're talking about a running vehicle the ignition system fails far more often. It's more likely your coils and/or caps are dying.

I saw a hack on youtube whereby valve seals can be changed without removing the head by stuffing the cylinder full of string at TDC. What are your thoughts on this practise?

No no no You use air pressure to do this. That is you make a custom hose that screws into the spark plug hole and connects to an air line. If you don't have a compressor one of those small 12v tire pumps will work. Leave it running because the air will leak past the rings.

I'm tired hanging onto the throttle for 10 minutes every time I start it up.

That's why the idle adjustment knob is so big. It is designed to be easily turned with your left hand while sitting on the bike at a stop sign. It's normal for these older bikes that don't have any type of high idle mechanism.

 

[Oliver Boy]

Thanks Dave, the compressed air sounds a way better idea than string.
Is there any hack you know of, to fabricate a tool for removing the keepers with the head on?
amc advocates removing the head -which is undoubtedly the most professional approach-but removing the engine is a whole new world of work ��...
I guess if a carb balance does not make a difference, coils will be the next fix.
Your comment about the idle adjustment makes me wonder if I am expecting too much from a 36 year old bike; but I'll have a good go

 

[Dave]

You have to remove the cams to remove the caps to get to the oil seals.

I'd at least pull the engine. Over the years I've learned the hard way it ends up being way easier, and in the end way faster (with a lot less cursing and blood), to move stuff out of the way instead of trying to work around it.

It's not really that big of a job other then the weight. Remove the seat, gas tank, air box, and the coils. Disconnect everything, pull the bolts, and set that baby on the ground. (PS The bottom right side of the frame comes off on yours) Unless you can can pickup and move 200lb objects around, get a friend to help.

Chapter 5 Engine Removal and Installation

OH! OH! Very important. Every valve shim is different. Either do only one at a time or get something like an empty egg cartoon, label it, and make 100% sure the shims, caps, and springs go back in exactly the same place.

 

[amc49]

The choke linkage actually DOES have a fast idle provision on it.

It is far easier to use a throttle stop on cold engine than repeat having to bend over to re-adjust the idle.

Don't use cord stuck in combustion chamber, these use rods with very small upper ends and they bend easily, in fact you may not even know you've done it. Only a hillbilly does that.