Gast 1550 driven by predator engine

[mountain man maple]

I am building two gas powered gast 1550 vacuum pump skids. Pumps will be belt driven to get required rotation for vacuum pump. The predator engine is a 3 hp 79cc model. Has anyone used this motor on a similar size vacuum pump? I'm looking for best speed to run engine and conserve fuel. Motor has most torque at 2500 rpm.

[madmapler]

I started out with the exact same set up 3 yrs. ago. I would go with a larger pulley on the motor. It doesn't take much power to run it. You can idle it down considerably. If I remember correctly, that's a 1700 rpm or so pump so you should halve the pulley size and then some. I later swapped to a bigger pump(2065) and still had plenty of power. The 79cc engine sips gas. I ran a 5 gallon gas tank and filled it up weekly. I still have it out in the shop and can measure the pulleys later if you like.

[mountain man maple]

Pump runs good at 1725 rpm I have a 5" pulley for one pump and a 6" for other. I think I'm going to replace 6" with 5" so everything would be interchangeable. I was thinking around a 4" on motor that would make engine speed a little over 2000 rpm. Is that still too fast?

[madmapler]

I just checked mine. Its 3.5" on the motor and 5" on the pump. I remember thinking I could go with a smaller pulley on the pump or larger on the motor just for the gas savings. It ran the pump pretty effortlessly. I also got more vacuum at a little higher rpms and I ran it that way for the season. I'm not suggesting it though but I personally would do it again. Watch the heat.

[MT Pockets Producer]

Lastwoodsman runs a couple of 1550 s with this set up also. He has a YouTube video that you can find if you search his posts. Don't know what size pulleys he is running though. We are setting one up but have not determined the pulley size yet. Was going to wait until we got the motor and do the math but it looks like madmapler has done that already. Think we will follow his lead especially with that fuel consumption, or lack thereof.

[MT Pockets Producer]

Madmapler, any issues starting the engine with your set up? Hooked ours up yesterday and had a real hard time getting the engine started when hooked to the pump. Starts on one pull when unhooked. We chose to try the same pulley diameters you used. I'm not sure if different size pulleys would make a difference or not. Any thoughts?

[WESTMAPLES]

make a idler pulley on a lever so when the pump is cold you leave the belt loose when starting the engine, then after a moment of engine running engage the idler so the belt is tight to turn the vac pump too

[pls009]

I was given a 1550 that I will attempt to run with a Predator motor. I took the pump apart because I could tell a bearing was getting bad. In the process, the thin gaskets are ruined. I see you can buy a repair/rebuild kit but would like to avoid that since I can buy the bearings at the local parts house and vanes seem great. My question: Has anyone just used a thin layer of RTV silicone gasket maker when putting these pumps back together? I'm guessing that I can't just buy the gaskets without buying the whole rebuild kit from GAST but could be wrong.

[madmapler]

I would think you would be fine doing that.

[pls009]

I would think you would be fine doing that.

Thanks! I'll give it a try

[mountain man maple]

Do you mean gasket for endplate or for the bearing cover plate? The pump endplates usually don't need any gaskets or sealant. I have 4 1550s one has to have a gasket on endplate for clearance. I don't use any gaskets on the bearing cover plates either.

[pls009]

Do you mean gasket for endplate or for the bearing cover plate? The pump endplates usually don't need any gaskets or sealant. I have 4 1550s one has to have a gasket on endplate for clearance. I don't use any gaskets on the bearing cover plates either.

I put a thin layer of sealer on end plates after replacing the bearings and the shaft spun easy. I installed the cover plate over the drive shaft and tightenened the three screws and it didn't allow the shaft to spin anymore. Loosen the screws a little and its fine again. Hmm. Are the clearances that tight? Wondering if i should remove sealant. I didn't see your post before putting back together.

[MT Pockets Producer]

No gaskets on pump end plates as mountain man stated. They are shims/spacers made of Mylar. 3rdgen gave me that advice when I worked on our first 1550. The shim in the rebuild kits are .003 inches. I think you can buy the shims separate from the kit but with the correct material you can make one. I had some Mylar window film that mic'd the same thickness so I made one with that. In fact we recently purchased a second 1550 and just yesterday I tore it down,cleaned, honed, replaced bearings and made another of those shims. It now pulls 26" of vac, the same as our other. Someone correct me if I am mistaken but too tight of a tolerance things could heat up and seize and too much space you would sacrifice vacuum. Also, someone gave advice that if the pump binds try a light tap on one end of the shaft or the other and it may adjust things. I have had that work as well.

[pls009]

Thanks for the reply! - the shaft binds right now so I will take it back apart and remove the RTV gasket material. When I took it apart, It had a bad bearing and must have got pretty hot, the shims very in tough shape and I don't really remember if there was one on each end or not. Could you tell me? Is there one on each cover plate or are they all different depending on the tolerances? The vac is at 26" so that is good but it seems to get hot fast and I'm sure it is related to the binding. Does anybody know what temp they typically run at when they are running properly at 1700 rpm's? I have a laser temp gauge from Harbor Freight - it is kind of a nice tool.

Does there need to be any shim or gasket on the bearing cover plates? I will call the local Gast distributor to check on the Mylar shims

[MT Pockets Producer]

Ours are lubricated models and get pretty warm to touch. You can hold your hand on them but it gets a little uncomfortable to keep it there. Does not seem to have been an issue as of yet though. The manual and rebuild kits show gaskets on bearing plates and one shim. I used gasket material I bought from autozone for those and I used a felt table/ chair leg pad for the washer on the shaft. Just need to shave the glue off with a razor. Both of our pumps had one purple shim on them. Gast shows them as .003 inches and that's what they mic'd at. I have the lubricated manual in an email from Gast. PM your email address if interested and I will send it to you.

[mountain man maple]

I don't use the shims when I rebuild mine. Mine run very hot probably in 150+ range. The only problem I have had is bearings wouldn't last. I bought quality bearings to try now. With cheap china bearings I had to change bearings once mid season.

[MT Pockets Producer]

I bought our bearings at Kaman Bearing locally and the guy there did me a favor and recommended the industrial bearings. He had the China bearings but told me for a few more bucks I would save myself a headache. At $10 a piece for the good ones I couldn't afford not to buy them.