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Petcock Question - Suzuki Tank on a Honda

Mr.humbucker

Mr.humbucker

Forum Mentor
Past Site Supporter
Recently a neighbour gave me a titled CM400T 1981 that frankly a dump might reject. Me being me saw a bike worth restoring and so for the next while I will be busy picking up the missing parts, rebuilding the carbs and seeing if she will fire up before I spend anytime or more money making her presentable. I envision making it a Frankenbike bobber of sorts.

My question is can I use a spare Suzuki tank from a GS550T and petcock for the rebuild? I recognize I will have to modify the mounts which won't be a big deal. The issue is the petcock on the Suzuki has a second part that I believe hooks up to the vacuum for the carbs? The Honda petcock for this bike doesn't have this vacuum port (or whatever it's called). So will my Suzuki tank and petcock work if I ignore this vacuum port?

Thanks in advance to anyone who replies a provides me with the proper terminology for the parts I don't know the names for.:D
 
Mr.humbucker said:
Recently a neighbour gave me a titled CM400T 1981 that frankly a dump might reject. Me being me saw a bike worth restoring and so for the next while I will be busy picking up the missing parts, rebuilding the carbs and seeing if she will fire up before I spend anytime or more money making her presentable. I envision making it a Frankenbike bobber of sorts.

My question is can I use a spare Suzuki tank from a GS550T and petcock for the rebuild? I recognize I will have to modify the mounts which won't be a big deal. The issue is the petcock on the Suzuki has a second part that I believe hooks up to the vacuum for the carbs? The Honda petcock for this bike doesn't have this vacuum port (or whatever it's called). So will my Suzuki tank and petcock work if I ignore this vacuum port?

Thanks in advance to anyone who replies a provides me with the proper terminology for the parts I don't know the names for.:D
Click to expand...

Just get a longer vacuum hose to suck on while you ride! Or put the petcock in the "Prime" position, that let's the fuel flow without vacuum.
 
Without a vaccuum line hooked up, the petcock can be used manually.
PRIme will be "on",
and
REServe and/or ON will be "off"
 
All of the above information is correct, but might not be quite complete. :-k

I don't think there was a 550T in 1980, which helps a bit. Most of the 1980 models had a petcock without a selector lever. Somehow, the shafties were spared that abomination. If the petcock on your 550T tank has a lever, it will work fine. Just point the lever down to park the bike, pull the lever to the rear to ride.

.
 
Is the vacuum the same on the vacuum port that controls the pet cock & the vacuum port used to synchronize the carbs? If so, just connect the Suzuki pet cock vacuum line to one of the carb. sync. ports on the Honda carb. I've never thought about it, but maybe.
 
Some of the Hondas don't have vacuum ports. :-k

The sync procedure calls for pulling a spark plug wire and noting the RPM drop. Repeat on the other side and compare.
dunno.gif


.
 
Some of the Hondas don't have vacuum ports
Click to expand...
...and many do. For vacuum gauges used to synch... Look near the manifold rubbers, on the engine...there are JIS screws...you could get an adapter to fit a tube, if you wanted to experiment

Depending on your carbs, one of these might be in use with a tube to the anti-backfire enrichment circuit ("newer" bikes had these to compensate for lean idle-mix...) but if you do have these, you can try without -Block the tube and enrich the idlemix

and repurpose one or both for your suzuki tap, as an experiment. But It should work fine.

Further, (or without any existing synch ports) you could get really interested and drill/tap a nipple into a carb's throat.
 
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Lots of direction and suggestions here. I will have a closer look at the carbs to see if there is/are sync vacuum ports and perhaps mcgyver a connection to one. And you are correct Steve, the GS550T was a one year model sold in 1981 ... my first bike new BTW. It has what I would refer to as the conventional Suzuki petcock of the day with the fuel and vacuum ports.

I will give all this a go once I have it all set up and the carbs finished .... waiting for a 118 jet in the mail and I need a few more fuel bowl screws to finish them up. If I can get her to run I am now leaning on buying an after market fuel tank that they gphave on eBay for bobbers etc. They're set up for the most part to fit old Hondas with perhaps a few modifications.
 
The carbs are Keihin for the Honda of course and it was missing one jet along with bowl screws (remember this was a free bike). The missing jet was a 118 size and I didn't have any Mikuni 118 jets on hand so I ordered one from Jets R Us online.
 
Few that routinely offer assistance here on the GSR site are more knowledgeable and quick to offer good guidance/input than Steve. So I suspect there is a reason for this question that he'll share with us in his reply.

I have a goal goal with this build, to do it with as much on hand parts and materials as I can. I am not afraid to spend a few bucks to make a build right but this time around I am hoping to clean out my parts bin to make it all come together. I have a set of progressive shocks that are perfect, a new 530 chain that I bought for another build that was too short that will fit the Honda Twin minus a few links and spark plugs (oh boy big savings there) NIB.

Tha tank is a bit if a challenge as it didn't come with one. I am leaning towards a cheap (I know ... never a good idea) after market bobber tank ... https://www.ebay.com/itm/Gas-Tank-S...a-CG125-Light-cured-Paint-Design/183957128997 Lots of measuring to do to ensure it will fit as the CM400T has a bit of a wide cross bar section.

I am thinking Matt or Gloss black with a fire engine red trim .... like the classic Batmobile color scheme ... but first I must get it to run! I get a kick out of pulling a junker off the rubish heap and making it look presentable and better yet a restored runner for years to come!

photos to follow.
 
That fleaBay tank might be a pretty good option. Crikey, that's less than a petcock. Had no idea these were available so cheap, but it makes sense. China and Taiwan crank out zillions of small-ish inexpensive motorbikes, so it stands to reason there are also huge factories cranking out zillions of small-ish gas tanks cheap.

Anyhoo, one thing to watch for if you use the Suzuki tank you have lying around is that old vacuum operated petcocks usually haven't been moved in decades, even on running bikes. It's quite common that moving the petcock lever starts a leak. It's also quite common to see that moving the petcock for the first time since the Reagan administration sends particles of mung down into the carbs, propping open the needles and causing the carbs to overflow.

So, uh, be ready for that sort of thing.
 
synchports-1.jpg

I looked it up. You should find synchports on that bike, just to the Right! of rubbers on the engine...one of these could drive your Suzuki fueltap.

Yes, "Nipples"- I think the ones that fit suzuki synchports should fit. They do on my Honda anyways.
 
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bwringer said:
That fleaBay tank might be a pretty good option. Crikey, that's less than a petcock. Had no idea these were available so cheap, but it makes sense. China and Taiwan crank out zillions of small-ish inexpensive motorbikes, so it stands to reason there are also huge factories cranking out zillions of small-ish gas tanks cheap.

Anyhoo, one thing to watch for if you use the Suzuki tank you have lying around is that old vacuum operated petcocks usually haven't been moved in decades, even on running bikes. It's quite common that moving the petcock lever starts a leak. It's also quite common to see that moving the petcock for the first time since the Reagan administration sends particles of mung down into the carbs, propping open the needles and causing the carbs to overflow.

So, uh, be ready for that sort of thing.
Click to expand...

Thanks for this. Ya this gas fleabay tank is really cheap, I suspect there are issues at this price but it may be worth a try. I will likely try the Suzuki tank first and jimmyrig the vacuum to the port that Gominrider (my fellow British Columbian) so kindly sourced for me. I will fork out for a new petcock OEMSuzuki if it fits the frame as I too have found the old ones are often troublesome. Hopefully I will get some time this weekend to see if it will fit and if I can get her to crank over and start. If it does I will put a Honda badge on one side of the tank and a Suzuki badge on the other. Hmmmm .... maybe a Yamaha seat and a Kawasaki set of lights ...
 
Mr.humbucker said:
The carbs are Keihin for the Honda of course and it was missing one jet along with bowl screws (remember this was a free bike). The missing jet was a 118 size and I didn't have any Mikuni 118 jets on hand so I ordered one from Jets R Us online.
Click to expand...
OK, I made a mistake, but it still works in principle. :-k
Somehow, I was hung up on the 550T reference, forgot that the engine in the bike was a Honda. :oops:
And there was mention of a Mikuni 118 jet.


Spyder said:
Just curious... What made you assume he wasn't? Sounds like a good project to fill in a little time. Good luck!
Click to expand...
That part is quite simple. Mikuni does not use "118" in their main jet numbering.
dunno.gif




Mr.humbucker said:
Few that routinely offer assistance here on the GSR site are more knowledgeable and quick to offer good guidance/input than Steve. So I suspect there is a reason for this question that he'll share with us in his reply.
Click to expand...
As listed above, but just like the TV commercial, "but wait, THERE'S MORE!!"

I don't know about the thread size difference between the Kehin and Mikuni jets, but the hole size of a Kehin 118 is the same as a Mikuni 105. The closest Mikuni jet to a 118 would be a 117.5, and its Kehin equivalent is 135, so you would have been running REALLY rich on that cylinder.

.
 
Steve said:
OK, I made a mistake, but it still works in principle. :-k
Somehow, I was hung up on the 550T reference, forgot that the engine in the bike was a Honda. :oops:
And there was mention of a Mikuni 118 jet.



That part is quite simple. Mikuni does not use "118" in their main jet numbering.
dunno.gif





As listed above, but just like the TV commercial, "but wait, THERE'S MORE!!"

I don't know about the thread size difference between the Kehin and Mikuni jets, but the hole size of a Kehin 118 is the same as a Mikuni 105. The closest Mikuni jet to a 118 would be a 117.5, and its Kehin equivalent is 135, so you would have been running REALLY rich on that cylinder.

.
Click to expand...

See ... I told you there was a reason! After reading the last paragraph twice, ok three times, I follow yer think'n, I think. I will let you know on the thread size, my suspicion is that they're the same.

You do raise a question for me here, if the two carb manufacturers use different sizing systems, is there a system/correlation to metric measurements for either? Hope that made sense.
 
Mr.humbucker said:
You do raise a question for me here, if the two carb manufacturers use different sizing systems, is there a system/correlation to metric measurements for either? Hope that made sense.
Click to expand...
I have read a paper on jet sizing, but don't remember some of the details. One manufacturer (I don't remember which one :oops:) numbers the jets based on the actual diameter of the hole. Another one numbers the jets based on the quantity of fuel that can flow through the hole in a given amount of time. And there are other methods of numbering, too. I have a chart that compares Keihin, Mikuni and DynoJet, they are all different.

.
 
Steve said:
I have read a paper on jet sizing, but don't remember some of the details. One manufacturer (I don't remember which one :oops:) numbers the jets based on the actual diameter of the hole. Another one numbers the jets based on the quantity of fuel that can flow through the hole in a given amount of time. And there are other methods of numbering, too. I have a chart that compares Keihin, Mikuni and DynoJet, they are all different.

.
Click to expand...

My first thought would be to size them by diameter if I were the manufacturer (no worries of that ever happening) but when I think about it it is all about the flow rate yet that could very on temperature and viscosity maybe? Good to know they're sized differently by manufacturer .... my day is complete now that I have learned something new about these bikes ... hey maybe that's a new category we can add ... GS Learning: Fact of the Day. 365 later we will all know a lot more.
 
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